Geological and Mineralogical Glossary

Geological and Mineralogical Glossary

Accretion: The accumulation of sediment by deposition, often occurring along a shoreline or in a river delta.

Acicular: A mineral growth formation consisting of fine needle-like crystals.

Adamantine: Having a brilliant luster like that of a Diamond.

Adit: A passage, or opening, driven into a mine from a hill or mountainside.

Adularescence: A white or silver blue iridescent haze displayed by moonstone and other gems. The adularescent sheen resembles a cloud of light that appears within the gem when it is turned to a certain angle.

Aggregate: A grouping of crystals. Or, a mineral habit where the crystals are packed closely together so they resemble grains, with the crystal structure not easily discernible.

Alkaline: An alkaline rock contains more than average amounts of potassium and sodium bearing minerals.

Allochromatic: Pertaining to color resultant from a mineral impurity, such as minor chemical substitutions or radiation damage.

Alloy: Combining two or more metals to increase durability, ductility, and other desirable properties.

Alluvial Deposit: An accumulation of dense mineral grains at the bottom of a sediment pile by the weathering action of a moving fluid such as water or wind. Or, gem deposits found in water after they have been separated from the mother or host rock.

Alpha and Beta Quartz: Quartz is the most stable and most common form of silica. Beta Quartz is only stable at temperatures above 1063º F (573º C). Thus, all Quartz specimens we see are Alpha Quartz. Once a sample of Beta Quartz is lowered below the above-mentioned temperature, it automatically transforms into Alpha Quartz. However, it preserves the original shape, but decreases in symmetry and adds some trigonal faces. Occasionally, in specific circumstances, the Beta Quartz transforms into Alpha Quartz without losing symmetry, and therefore does not add the trigonal faces. Such specimens are shaped as bipyramidal hexagons, and are sold by dealers as "Beta Quartz". They obviously cannot be Beta Quartz at the current temperature, but are Alpha Quartz paramorphs of Beta Quartz.

Amber: A prehistoric fossil resin that usually has plants and other debris, including insects trapped inside which then hardens over thousands of years, usually found underground in areas what used to be deep forest vegetation.

Ambroid ~ Amberoid: Manmade reconstructed or pressed Amber. It is produced by melting small bits of Amber together under pressure.

Amorphous: Not having form. There is no internal or regular crystalline structure, such as with Amber or Jet.

Amphibole: A mineral group that consists of common, dark-colored, rock-forming silicate minerals, such as Hornblende, Tremolite, and Actinolite. Some fine crystals can be rather fibrous, almost asbestos-like.

Amulet ~ Talisman: An object sometimes fashioned and engraved with a symbol that is believed to provide magical, medicinal, or protective power. Synonym: Talisman.

Amygdule ~ Amygdale: A formation which occurs when the gas bubbles or vesicles in volcanic lava or other extrusive igneous rocks are infilled with a secondary mineral such as Calcite, Quartz, Chlorite or one of the Zeolites.

Anhedral: This refers to a crystal with no well-formed external faces.

Anhydrous: Lacking any water.

Anion: A negatively charged ion that is attracted to the positively charged anode of an electrolyte cell.

Appraisal: An evaluation performed by a licensed gemologist to determine the value of a gem or item of jewelry.

Arborescent: Minerals having a treelike form with branches similar to the ones on a tree.

Argillacious: A type of material, or matrix, which is mostly composed of clay.

Asbestiform: A mineral with fibers looking and feeling like those of Asbestos. It is a particular kind of fibrosity in which fibers have high tensile strength and flexibility.

Asbestos: A group of naturally fibrous silicate minerals, such as Chrysotile.

Ash: (Volcanic) A harsh abrasive type of ash that is made up of small rocks, minerals, and volcanic glass fragments.

Assay: A test of the purity of an alloy. Official assay offices determine whether a piece qualifies for an appropriate hallmark.

Asterism ~ Star: A star-like luminous effect that reflects light in some gemstones, such as Sapphire, Ruby, Garnet, and Sunstone. The majority of Stars are either 4 rayed or 6 rayed. The rays will intersect. They are usually caused by fibrous inclusions of Rutile.

Atmospheric Shock Wave: An atmospheric shock wave is compressed air that is formed by a volcanic eruption.

Atom: An atom is composed of protons, electrons, and neutrons.

Aventurescence: A colorful play of glittering reflections of small plate-like inclusions, such as Hematite, Goethite, or Fuschite. This is not to be confused with Play-of-Color.

Axis: (Crystal Axis) A reference direction in a crystal that is parallel to symmetry directions or the intersection of faces. Plural: Axes.

Ballistic Fragment: A piece of rock that is ejected from a volcano with great speed.

Banding ~ Banded: The presence of color zoning lines, or bands, in or on some minerals, such as Agate and Malachite.

Batholith: A batholith is a huge body of igneous rock that solidified under the Earth but now has at least 100 square kilometers (40 square miles) exposed. Half Dome at Yosemite National Park, California, USA, is a granite batholith. Mt. Rushmore was carved from the northeastern edge of the Harney Peak Granite Batholith.

Bi-Colored: A mineral specimen or gemstone with two distinct colors which are clearly separated from each other. This is most commonly seen in Tourmaline and Ametrine.

Biomass: The amount of animal and plant life in an area. Biomass is usually expressed in units of weight per volume.

Birefringence: It is the magnitude of the difference in the refractive indices of birefringent minerals.

Birefringent ~ Doubly Refractive: Having two or three refractive indices, a characteristic of minerals not possessing cubic symmetry.

Bituminous: Containing much organic, or at least carbonaceous matter, mostly in the form of the tarry hydrocarbons, which are usually described as bitumen. This term also pertains to bituminous coal.

Bladed: A mineral growth pattern appearing elongated and flattened.

Block: (Volcanic) A volcanic block is a chunk of rock (over 64 mm) that is ejected (thrown) from a volcano. Blocks usually originate from the volcano's cone and are solid chunks of older lava flows. A block is one size of tephra.

Blocky: Minerals which are block-like in appearance. This type of crystal habit can be found in minerals such as the feldspars, which can nearly be as thick as they are long.

Blowdown: An area of trees that has been blown over by a volcanic blast.

Bort: A term for industrial grade diamonds.

Botryoidal: A mineral growth pattern resembling bubbles or a cluster of grapes.

Breccia: Broken rock held together by a clastic cement or fine grained material.

Brilliance: The degree to which a faceted gem sparkles and returns light from within; dependent upon the gem’s cut and refractive index. Synonyms: Life, Liveliness

Brittle: Breaks or powders easily.

Bubbles: Spherical or tear-shaped bubbles of gas captured in glass stones. Bubbles can also be found in resins (like plastics and amber), and much less-frequently in minerals.

Butte: A flat-topped rock or hill formation with steep sides.

Cabochon: A gem cut style distinguished by its smooth convex top and no facets. It is flat on the bottom. It is the most common form of gem cutting.

Cabochon, Buff Top: It has the smooth convex top of a cabochon with a faceted bottom. It is a relatively new cut.

Caldera: A large depression formed from a collapsed volcano. Calderas are often circular or elliptical.

Carat: The standard unit of gem weight (mass); 1 ct. = 0.2 grams.

Carbonaceous: Referring to a rock or sediment that is rich in carbon. It also refers to a sediment containing organic matter.

Cataclastic Rock: Metamorphic rock produced by the crushing and grinding of pre-existing rocks, which are still visible as crushed fragments.

Cation: A positively charged ion that is attracted to the negatively charged cathode of an electrolyte cell.

Cave: A deep hollow or natural passage under, or into, the earth that has one or more openings to the surface.

Cavern: A large cave.

Chatoyancy ~ Cat’s Eye: A Cat’s Eye appearance when a stone is illuminated. It is caused by parallel arrangement of tiny needle-like inclusions within a crystal.

Chemical Formula: The standard way of stating the chemical composition of a mineral in terms of the number of atoms of each element contained in that mineral.

Chert: This is a hard, dense, fine-grained type of sedimentary rock, a crystalline aggregate of silica. It was formed from deposits of silica-based skeletons of microscopic marine organisms and other organic matter. Black Chert is called Flint.

Cinder Cone: A cone-shaped volcano. Its steep sides are formed by volcanic cinders that fall to the Earth close to the vent.

Cinders: Small fragments of lava that are about 1/2 inch (1cm) across.

Clastic: Consisting of fragments of minerals, rocks, or organic structures that have been moved individually from their places of origin. A synonym for Detrital.

Cleavage: The tendency of a mineral to break along a plane due to a direction of weakness in the crystal.

Cluster: A group of crystals showing an underlying order in their placement. This order may be either two or three dimensional.

Cockscomb ~ Coxcomb: This term refers to an aggregate composed of flaky or tabular crystals that seem adjoined from a base, with grooves between long, slender, arc-like crystals.

Colloform: Formed of ultra-fine grained particles into a reniform or botryoidal shape. It is usually finely banded from variations in the content of the suspended material forming it.

Colluvial Deposit: A loose deposit of rock debris and sediment which has been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity. The deposits that collect at the foot of a steep slope or cliff are also known by the same name.

Color: An important property when evaluating a gem. The quality of a gem can be measured based on either the presence or the absence of color.

Color Change: A “color change” stone changes color between two really obvious different colors. **For more details on pleochroic and color changing gems and minerals, click here: Pleochroism

Color Shift: A “color shift” stone will show a change of hues within the same basic color family, such as from yellowish green to grass green, or from yellowish red to brownish red. It is not really a color change, just a “shift” in color. **For more details on pleochroic and color changing gems and minerals, click here: Pleochroism

Columnar: An aggregate defined as a mineral which has parallel, slender, compact, adjoining crystals.

Composite Volcano: A composite volcano is a volcano that has a steep volcanic cone that is built up by dense lava flows and pyroclastic debris.

Concentric: An aggregate described as having foliated masses that are somewhat spherical and rotate about a center; appearing like a rose (rosette). It is also used to describe a form of banding where the bands are circular, forming rings about a central point.

Conchoidal: Shell-shaped; the more compact rocks, such as flint, which break with concave and convex surfaces, are said to have a conchoidal fracture.

Concretion: A hard, compact mineral-mass of mineral matter that forms usually in sedimentary rock around a center such as bone, shell, leaf, or fossil.

Conduit: (Volcanic) A passage through which magma (molten rock) flows in a volcano.

Conglomerate: A cemented clastic rock containing rounded fragments of pebble size gravel; the consolidated equivalent of gravel. The composition of rock or mineral fragments may vary widely in size, but are usually rounded and smoothed from transportation by water or wave action.

Continental Drift: This refers to the movement of the Earth's continents. The land masses are hunks of Earth's crust that float on the molten core. The Earth was once one large supercontinent called Pangea, or Pangaea. **For more detailed information and maps, click here: Pangea ~ Pangaea

Continental Plates: The crust of the Earth is broken into plates. The plates are enormous chunks of rock that float atop the soft mantle. The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Continental plates are thicker, older, and less dense than oceanic plates. These plates are about 125 kilometers thick and are made of granite that is about 3 billion years old.

Continental Shelf: The continental shelf is the part of the ocean floor next to each of the continents. The sea floor slopes gradually from the continent to a depth of about 650 feet (200 m). Beyond the continental shelf the sea floor drops steeply.

Contra Luz: This term refers to a color phenomenon found in some transparent Precious Opals. They display a brilliant play of iridescent colors only when light shines through the stone. When the light is on the same side as the viewer, the iridescence is not readily seen. Unlike other Opals, Contra Luz Opals are usually faceted to enhance this beautiful display of colors.

Copal: A type of resin produced from plant sap, often from members of the genus Copaifera. It is a resinous substance in an intermediate stage of polymerization and hardening between more "gummy" resins and Amber. It is geologically younger than Amber. It is commonly found with inclusions of insects and/or vegetation debris.

Coprolite: This is fossilized animal excrement embedded in rock. Colorful specimens of dinosaur coprolite are commonly found and polished.

Coralloidal: Having the form or appearance of Coral.

Core: The innermost layer of the Earth. It consists of iron-nickel; it is under great pressure and is very hot. The inner core is solid; the outer core is molten.

Crater: A circular depression in the ground. It has steep sides and contains a volcanic vent.

Crust: The Earth's crust is its outermost rocky layer.

Cryptocrystalline: Being constituted of submicroscopic crystals, such as Chalcedony. Agates and Jaspers are also classified as cryptocrystalline quartz.

Crystal: A solid body having a regularly repeating arrangement of its atomic constituents; the external expression may be bounded by natural planar surfaces called “faces.”

Crystal Face: The flat exterior surface of a crystal.

Crystalline: Having the properties of a crystal: a regular internal arrangement in three dimensions of constituent atoms.

Crystallization: A liquid to solid phase change.

Crystallography: The study of crystals, including their growth, structure, physical properties, and classification by form. **For more details on crystal systems, click here: Crystal Systems

Cubic: (Crystal System) Defined by three mutually perpendicular axes of equal length—the highest symmetry class.

Cultured Pearls: These Pearls are produced by inserting a Mother-of-Pearl bead into a live oyster and then returning it to the water, usually into "farming beds" to grow its nacre coating.

Dacite: A type of volcanic rock that is light-colored and rich in silica.

Decomposition: Some minerals may disintegrate or decompose due to a molecular breakdown. It is unpredictable and occurs randomly most commonly in Sulfide minerals.

Deliquescence: The opposite of efflorescence. Water gets absorbed into the structure of a mineral, causing it to dissolve.

Dendrites: The presence of tree-like, or fern-like inclusions. They are confined to thin layers and produce interesting and unique effects. They are deposits of minerals, such as Manganese oxides, that precipitated along thin fractures of the host mineral.

Dense: Compact, fine-grained, lacking pore space. A rock or mineral with a high specific gravity.

Density: See Specific Gravity.

Deposit: Earth material of any type that has accumulated by some natural process and is large enough to invite exploration, such as a mineral or ore.

Detritus: Very small, loose fragments of rock caused by weathering, mechanical breakage or blasting of rock and mineral masses.

Devitrification: This is a process in which natural glass substances change their structure into that of crystalline solids.

Diabase: A mafic, holocrystalline, intrusive igneous rock equivalent to volcanic basalt or plutonic gabbro.

Diamagnetism: A magnetic property which causes a mineral to be repelled from magnetic fields.

Diatreme: A volcanic vent or pipe that is formed by gas-charged magma.

Dichroism: The ability of some gems to display a second color when viewed from a different angle. **For more details on pleochroic and color changing gems and minerals, click here: Pleochroism

Diaphaneity: The degree of transparency, or ability to pass light, without interfering with that passage.

Diffraction: A modification which light undergoes especially in passing by the edges of opaque bodies or through narrow openings and in which the rays appear to be deflected.

Diffusion Treatment: Diffusion treated stones are color-enhanced stones. The diffusion process only colors the outer surface of the stone, so chipping or repolishing will result in a loss of color. Diffusion-treated stones are already-cut stones that are heated in the presence of other compounds that will infuse the extreme outer surface stone with color.

Dike: A tabular body of igneous rock injected into a fissure when molten and cuts across the structure of the adjacent rocks.

Dispersion: The systematic variation of refractive index with color in a substance; colors separate during refraction of white light. It leads to “fire” in a gem. Or, more simply, the splitting of light as it enters a gemstone. **For a list of the Dispersion values of various gemstones, click here: Dispersion Values

Disseminated: Fragments of minerals dispersed in a rock.

Divergent: A radiating crystal habit.

Dome: A lava dome is a dome-shaped mound that is formed around a vent by viscous lava.

Dormant Volcano: A volcano that is not active now, but may erupt in the future.

Double Refraction: A phenomenon which occurs when the ray of light entering a crystal is refracted and divided into two rays instead of one. This is most commonly seen in Calcite and Zircon.

Doublet: A stone made of two components, generally held together with a clear adhesive. Most commonly found with Opals.

Drip Stones: Stalagmites or stalactites.

Druse ~ Drusy: (Druze ~ Druzy) A cavity in a rock or vein with walls encrusted with small projecting crystals.

Ductility: The ability of a metal to be hammered or drawn.

Dull: Minerals in which there is a total absence of luster, as with chalk or kaolin.

Effervesce: To bubble or hiss, as in carbonated water.

Efflorescence: The growth of mineral crystals or crusts on the surface of rock in a mine, or elsewhere, caused by the evaporation of the water which that mineral is dissolved in.

Ejecta: Material blown out of a volcano during an eruption.

Electron: Negatively charged particles surrounding the nucleus of an atom.

Element: A substance composed of atoms bearing an identical number of protons in each nucleus. An element cannot be decomposed into other substances (except by radioactive decay or bombardment with high-speed particles). An element's structure is made up of only a single type of atom which is 100% pure, containing no other substances.

Emery: A rock which is a mixture of granular corundum and other minerals. It usually contains spinel and/or iron oxides, such as magnetite and hematite.

Enhanced Stones: Stones that have been treated to improve their color, clarity, finish, strength, or other characteristics. Some common enhancements are heat-treatment, irradiation, coating the surface, filling cracks, oiling, surface diffusion (coating the surface then applying heat), bleaching, dyeing, etc. **For more details on gemstone treatments, click here: Gemstone Treatments

Enhydro ~ Fluid Inclusion: A mineral or crystal containing a pocket of water, or other liquid. The crystal or mineral grows around the liquid deposit.

Epitaxial Growth: A crystal of one mineral grows on or around a crystal of another mineral.

Epoch: An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years. **For more details on the Earth’s geologic time scale, click here: Earth's Geologic Time Scale

Equant: Crystals having the same, or nearly the same, diameter or the same dimensions in all directions.

Era: Two or more geological periods comprise an era, which is hundreds of millions of years in duration. **For more details on the Earth’s geologic time scale, click here: Earth's Geologic Time Scale

Eruption: (Volcanic) An eruption is volcanic activity in which lava, tephra, or gases are released.

Euhedral: Showing no outward crystal form. The growth of the crystal was bounded by contact with neighboring crystal growth, preventing the crystal faces from showing.

Evaporation: The change by which a substance is converted from a solid or liquid state into a vapor state.

Exsolution: A process which occurs when a homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. In most cases it occurs upon cooling, when the temperature is below that of mutual solubility or the stability of the solution.

Extinct Volcano: A volcano that is not likely to erupt again.

Extrusive: A term used to describe igneous rock that has been erupted onto the earth's surface.

Facet: The cut and polished part of a gemstone.

Faceting: Cutting a gemstone to have multiple polished and angled flat surfaces. This is done to bring out the brilliance of a gem.

Ferromagnesian: Containing iron and magnesium. Applied to certain dark silicate minerals, especially Amphibole, Pyroxene, Biotite, and Olivine.

Ferruginous: This refers to a material containing iron, or having an iron-like appearance.

Fiber: The smallest single strands of asbestos or other fibrous materials.

Fibrous: Applied to minerals that occur in fibers, such as Asbestos. Or, consisting of fine threadlike strands.

Filiform: A mineral occurring as thin threads, often twisted like the strands of a rope, such as Native Copper.

Findings: Metal parts used in jewelry construction and repair, such as bails, posts, and ear nuts.

Fire: The division of colors in a colorless transparent gem such as Diamond due to dispersion. White light disperses into a rainbow of colors.

Fissure: A fissure is a crack in a rock. A volcanic fissure is one from which lava erupts.

Flaw: An imperfection in a gemstone. Flaws include: cracks, inclusions of other minerals or liquid-filled cavities. Flaws can greatly reduce the value of a stone, but in some cases, like moss agate or rutilated quartz, the flaws increase the value of the stone.

Fluorescence: An instantaneous optical effect arising from the movement of electrons within the structure of a material. It is most commonly observed by exposure to shortwave and/or longwave ultraviolet lamps. Or, the emission of visible light by a substance exposed to ultraviolet light. **For more details on fluorescence, click here: Fluorescence

Fluvial Deposit: Mineral deposits found in rivers.

Fold Mountains: A type of mountain range that is formed when two continental plates collide (or one continental plate colliding with an oceanic plate). The colliding crust is compressed and pushed upwards (uplifted), forming mountains. For example, the Himalayas were slowly formed when the Indian plate collided with the Asia-European plate millions of years ago.

Foliated: Substances made up of thin leaves, like Mica.

Fossil: The remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities. Fossils come in many different mineral and organic forms, including plain-looking rocks, marble-like casts of ancient animals, opals, and amber.

Fossil Fuel: Petroleum (oil), natural gas and coal are fossil fuels, organic materials that are high in energy. Fossils fuels are formed in a process that takes millions of years. The organic material (dead plants and animals) is covered by layers of sediment, then heat, pressure, and bacterial action change the material into pools of oil and gas (or are compressed as coal).

Fracture: A break with an uneven or irregular surface crack.

Friable: This describes a mineral which crumbles easily.

Fulgurite: Fulgurites form naturally by the very high temperature melting of quartz sand or bare rock during a lightning strike. The result is an irregular, branched, and often foamy hollow tube of silica glass called a Fulgurite. Fulgurites may be referred to as "Petrified Lightning." Fulgurites are found worldwide in desert sands. The largest Fulgurites have been found in the sands of Florida in the USA. Also see Lechatelierite

Fumarole: A vent hole in a volcanic area from which hot smoke and gases escape, and around which some minerals, such as sulfur may condense.

Gabbro: A coarse, dark, intrusive, igneous rock. The crystals are large enough to see without a microscope. Chemically, gabbro is the same as basalt (gabbro forms when magma cools below the surface of the Earth, but basalt forms when the lava cools on the surface).

Gangue: Minerals with no metal ore value. Waste rock that is placed in the mine tailings dump.

Gem: A natural mineral, crystal, or gemstone that has been fashioned to enhance its natural beauty. The term “gem” or “gemmy” may also be applied to a mineral or crystal of excellent quality, such as one with outstanding color and clarity.

Gemologist: A person who has successfully completed recognized courses in gemology and has proven skills in identifying and evaluating gem materials.

Gemology: The study, science, art, and profession of identifying and evaluating gemstones.

Gemstone: A substance that has beauty, durability, and rarity and that can be fashioned into an item of personal adornment.

Geode: A sphere or oval shaped hollow or solid rock cavity containing crystallized minerals. Mineral crystals which may be found in Geodes are Quartz, Agate, Calcite, Peridot, Celestite, and many more.

Geological Time: The history of the Earth is described in geological time, which is measured in millions of years and billions of years. The divisions used are: eon, era, period, and epoch. **For more details on the Earth’s geologic time scale, click here: Earth's Geologic Time Scale

Geothermal: Heating due to the increasingly high temperatures that occur as the depth below surface level increases. Geothermally heated water is responsible for geysers and other hydrothermal processes.

German Silver: An alloy of nickel, copper, and zinc. It does not contain any silver.

Geyser: A natural hot spring that intermittently ejects a column of water and steam into the air. This is the most commonly seen hydrothermal process.

Glacier: A river of accumulated of snow, compacted by gravity and weather into ice, often found on the upper slopes of mountains. Their movement is extremely slow, usually measured in centimeters, or meters, per day.

Glassy: Igneous rocks that have no crystals, only super-cooled magma, such as Obsidian.

Globular: A smooth rounded surface of crystalline intergrowth.

Gneiss: A high grade metamorphic rock that has been subjected to more heat and pressure than schist. It is coarser than schist and has distinct alternating banding of feldspar, mica, quartz, hornblende, muscovite, biotite, and garnet. Gneiss can be formed from sedimentary rock or from the metamorphism of the igneous rock.

Gossan: An iron-bearing weathered product overlying a sulfide deposit. It is formed by the oxidation of sulfides and the leaching-out of sulfur and most metals, leaving hydrated iron oxides and rarely sulfates.

Grain: A mineral or rock particle which is less than a few millimeters in diameter and generally lacking well-developed crystal faces, such as a grain of sand.

Granitic Magma: A type of molten rock from which granite is formed.

Granular: Consisting of grains of approximately equal size and ranging from two to ten millimeters.

Greasy: A definition of minerals which are oily to the touch or sight.

Greenhouse Effect: An increase in the temperature of a planet as heat energy from sunlight is trapped in the atmosphere. Excess carbon dioxide and water vapor increase this effect. The greenhouse effect is strong on Earth and Venus, maintaining warm temperatures.

Group: (Mineral Group) A set of minerals that share the same crystal structure. **For more details on the mineral group classifications, click here: Mineral Groups

Habit: A characteristic shape of a mineral, either a crystal shape or the shape and style of polycrystalline intergrowths.

Hardness: Resistance to abrading or scratching; measured from 1 to 10 on the Mohs Hardness Scale. **For a list of the Mohs Hardness values of various minerals and gemstones, click here: Mohs Hardness Scale

Heat Treatment: The application of heat to a gemstone for the purpose of improving its depth, clarity, or richness of color. The most routinely heat treated gem is Sapphire. Chemicals may also be used in conjunction with the heat treatment. Be aware of the true treatment the gem has undergone. There are many unscrupulous dealers out there!

Hexagonal: (Crystal System) Defined by three equal axes lying in a plane and intersecting at 120 degree angles and a fourth perpendicular axis that is a six-fold rotation.

Hopper Crystal: A crystal with a “hoppered” shape. The edges of “hoppered” crystals are fully developed, but the interior spaces are not filled in. The interior sections never filled in because the crystal was growing so rapidly that there was not enough time or material to fill in the gaps. They are also known as Skeletal, Fenster, and Window crystals.

Hornfels: A fine-textured metamorphic rock formed by contact metamorphism. Contact metamorphism occurs when a mass of hot magma intrudes into pre-existing rock. Rock in close proximity to the magma is temporarily softened or melted and recrystallizes with an altered texture, producing a hornfels. The term hornfels is often restricted to rocks produced by contact metamorphism of shale, slate, or mudstone.

Hot Spot: A hot spot is an area in the Earth's lithosphere through which magma (molten rock) rises. Volcanoes often erupt over hot spots. This term also applies to dark smoky spots found on some quartz crystals which have been exposed to natural radiation at some time in their growth cycle.

Hydrocarbons: Organic substances that are composed only of hydrogen and carbon.

Hydrologic Cycle ~ Water Cycle: The journey water takes as it circulates from the Earth to the sky and back again.

Hydrothermal Veins: Mineral veins which are formed at relatively high temperatures (300-500 degrees Celsius) and at relatively great depth.

Hydroxides: Compounds of metallic elements combined with water and hydroxyl.

Hygroscopic: Readily absorbing water, especially from the air.

Ice Age: A time lasting thousands of years during which the Earth is very cold and largely covered by ice and glaciers.

Idiochromatic: Color is inherent and due to some aspect of chemical composition and crystal structure.

Igneous Rock: Rock which was formed by the solidification of magma from a volcano. It can be either volcanic rock or plutonic rock. See the individual listings.

Imitation: A substance that simulates a genuine gem, although typically applied to glass, plastic, and other non-crystalline materials.

Inclusion: An inclusion is material trapped within the body of a crystal which is different from the primary elements of the host crystal. Inclusions are generally other minerals, but they can also be water, gas, or petroleum based. In some cases they may even be organic in nature.

Incrustation Pseudomorph: This is a type of pseudomorph which occurs when one mineral forms a coating over another mineral, causing the coated mineral to dissolve. A hollow cast of the mineral that coated the dissolved mineral remains. Also see Pseudomorph.

Infiltration Pseudomorph: This is a type of pseudomorph which occurs when some atoms of a mineral are replaced by different atoms, forming another mineral. Also see Pseudomorph.

Intergrowth: A composite of two or more crystals in intimate contact resulting from their simultaneous crystallization.

Intrusion: An intrusion is a body of igneous rock that has crystallized from molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Roman god of the underworld. Correspondingly, rocks of this kind are also referred to as igneous plutonic rocks or igneous intrusive rocks. This is to be contrasted with extrusive rocks.

Ion: An atom, or group of atoms, that have acquired a net electric charge by the gain or loss of one or more electrons.

Iridescence: A play of color in a mineral, crystal, or gemstone resulting from inclusions or layers of other minerals, either internal or on the surface. Limonite deposits or inclusions frequently exhibit an iridescent rainbow of colors.

Irradiation: The act of being exposed to radiation. Many stones are irradiated in order to enhance their color. Being irradiated changes the crystal structure of the mineral by moving electrons. Irradiation techniques bombard the crystal with high-energy radiation, producing a stone with very little radioactivity and a change of color. Some color changes caused by irradiation are permanent, others are unstable and can be reversed by heating or exposure to sunlight.

Isomorphous: This refers to two minerals having the same molecular arrangement, or crystal form, while being composed of different elements.

Jasperoid: This name applies to rocks that consist mainly of silica and have been formed by replacement or metasomatic alteration.

K-T Extinction: The mass extinction that occurred 65 million years ago, at the boundary of the Cretaceous and Tertiary periods.

Labradorescence: Iridescence in metallic hues, like Schiller, found especially in Labradorite and Spectrolite. Blues and greens are most commonly seen, but the whole color spectrum can be found in these minerals.

Lahar: A landslide or rapidly moving mudflow, or the deposit left by one, of volcanic ash and fragments mixed with water. It occurs on the flanks of a volcano, or the stream beds and channels below it.

Lamellar: A growth pattern consisting of thin layers, plates, or scales.

Lapidarist: A person who is a cutter, polisher, or engraver of precious stones.

Lapidary: The art and practice of cutting gem stones and minerals.

Lapilli: Pieces of rock or lava that range from 2 and 64 millimeters across. Lipilli are thrown into the air by volcanic explosions.

Lateral ~ Side Blast: (Volcanic) A volcanic eruption that occurs on the side of a volcano; the energy of the blast is directed horizontally.

Lechatelierite: An amorphous silica glass which can form in several different ways. It forms naturally by the very high temperature melting of quartz sand or bare rock during a lightning strike. The result is an irregular, branched, and often foamy hollow tube of silica glass called a Fulgurite. Fulgurites may be referred to as "Petrified Lightning." Fulgurites are found worldwide in desert sands. Lechatelierite also forms naturally as the result of high pressure shock metamorphism during meteorite impacts and volcanic explosions. Lechatelierite is also a common component of a type of glassy ejecta called a Tektite. Lechatelierite may also form artificially. One example is the Trinitite produced by the melting of quartz sand at the first nuclear bomb explosion at Trinity Flats, White Sands, New Mexico. Another way it is artificially produced is when a high voltage electrical distribution network breaks and the lines fall onto a conductive surface with sand beneath. Lechatelierite is a mineraloid, as it does not have a crystal structure. Although it is not a true mineral, it is often classified in the quartz mineral group. Also see Fulgurite and Tektite.

Leptite: A quartz-feldspathic metamorphic rock that is fine-grained with little or no foliation; formed by regional metamorphism of the highest grade.

Lignite: A low-grade brownish black coal.

Limb Cast: Agate is deposited in the vacant cavities of tree branches once covered by hot volcanic ash. The wood itself burned away and left the vacant cavities. The original form of a tree limb is maintained. Some casts may show bark and/or wood textures, or become opalized.

Limestone: A sedimentary rock which is composed of calcite that came from the beds of evaporated seas and lakes. It also contains skeletal fragments of marine organisms.

Lithophysa ~ Lithophysae: A small cavity, or cavities, found in felsic volcanic rocks believed to be caused by expanding gases in tuffs before solidification. If it has become lined with crystals it may be referred to as a geode or a thunderegg. The word lithophysae is from the Latin for "rock bubble."

Lithospheric Plates: A series of slabs that make up the Earth's hard outer shell. There are 16 major lithospheric plates that float on softer layers of the Earth's mantle.

Littoral Zone: (Intertidal) This zone is where the sea meets the land.

Lode: A deposit consisting of several veins spaced closely enough so that all of them, together with the intervening rock, can be mined as a unit.

Loess: A deposit of tiny windblown particles.

Luminescence: A collective term for the emission of visible light under the influence of certain rays (other than incandescent and daylight), as well as by some physical or chemical reaction, but not including pure heat radiation. It is most commonly seen as fluorescence under ultraviolet lighting. **For more details, click here: Fluorescence

Luster: The manner in which a substance reflects light from its surface; it is affected by the surface’s smoothness and the substance’s reflectivity.

Macle: This term refers to a twinned or double crystal.

Mafic: Dark colored rock; usually applied to igneous and metamorphic rocks containing magnesium and iron rich minerals.

Magma: Mobile molten rock material from which igneous rocks form by solidification. It is also called as “liquid rock.”

Magnet: This is described as an object that is surrounded by a magnetic field, thereby causing iron or steel materials to be attracted to it.

Magnetic: Materials which give off magnetic fields, or materials that are attracted to magnetic fields. **For more detailed information on magnetic minerals, please click here: Minerals With Magnetic Properties

Magnetic Field: This refers to an area encompassing a magnet or electric current which has the ability to attract or repel certain objects anywhere within the field.

Magnetic Field (Earth's): The Earth's magnetic field is aligned with the north and south poles, and has reversed many times during geologic history.

Magnetism: This is the ability of certain metals to attract other metals.

Malleable: Having the ability to be hammered into thin sheets without breaking, such as with gold or silver.

Mammilary: An aggregate described as being smooth, rounded, agglomerations. Rounded agglomerations of mammilary aggregates are larger than reniform agglomerations and considerably larger than botryoidal agglomerations.

Mantle: The layer of the Earth located between the crust and the molten core.

Mass Extinction: The process in which huge numbers of species die out suddenly. The dinosaurs and many other species went extinct during the K-T extinction, which was probably caused by an asteroid colliding with the Earth.

Massif: A large mountain, or group of connected mountains, that are a part of, but independent from, a mountain range.

Matrix: A mineral or mineral compound within, or upon, which another mineral or mineral compound occurs.

Matter: Anything which has mass and occupies space.

Melee: A small faceted diamond, under .20 carats.

Metacryst: A large crystal developed in metamorphic rock by re-crystallization from another mineral.

Metallic: Minerals having the luster of a metal.

Metallurgy: The science and art of separating metals and metallic materials from raw ore by mechanical and chemical means. This term also applies to the study of the physical properties of metals as affected by composition, mechanical working, and heat treatment.

Metamict: The disruption of the crystalline structure at the atomic level by radiation, yet retaining the original macroscopic crystal form or shape. This is common among radioactive minerals, and those minerals that are found close to them.

Metamorphic: Rocks which have been altered by heat or intense pressure at a depth in the earth's crust causing new minerals and new structures in the rock to be formed.

Metasomatism: The alteration of the chemical composition of a rock or mineral by interaction with dissolved chemicals and gases in capillary fluid. This causes the mineral to be changed into another mineral without melting, and usually with little or no change in volume or shape.

Meteorite: A meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.

Meteoroid: Tiny stones or pieces of metal that travel through space.

Miarolitic: Igneous rock that has many small, irregular cavities. These cavities are usually more angular and irregular than those of a vug.

Micaceous: Material composed of or resembling Mica. It occurs in thin plates or scales like Mica.

Microcrystal: Crystals of less than 0.01 millimeters in their greatest dimension. These crystals, usually occurring in masses, are so small that only their speckle reflections are all that betray their existence at 30X magnification.

Micromount: A natural mineral specimen requiring magnification for proper observation. The specimen should contain distinct crystals.

Milling: The first stage of mineral processing in which the ore pieces from the mine are further mechanically reduced in size to maximize efficiency of the concentration process.

Miner: A person who professionally excavates and searches for gemstone and mineral specimens.

Mineral: A natural geologically occurring substance (usually inorganic) that is crystalline and has a composition that can be defined by a simple chemical formula. Or, solid matter composed of elements in specific combinations and arrangements.

Mineralogist: A person who studies the formation, occurrence, properties, composition, and classification of mineral specimens.

Mineralogy: An earth science focused on the chemistry, crystal structure, and physical properties of minerals, their distribution and identification.

Mineraloid: A mineral-like substance that does not have a crystal structure and possesses a chemical composition that varies beyond the accepted range for a specific mineral.

Mine: An underground area or cave where precious gems, semi-precious gems, and minerals are excavated.

Mohs Hardness Scale: See Hardness.

Molecule: A molecule consists of two or more atoms chemically bonded together. It is the smallest particle, composed of two or more similar or different atoms, into which an element or a compound can be divided down to without changing its chemical and physical properties.

Monoclinic: (Crystal System) Defined by three nonparallel axes where there are only two right angles between the axes and no high-order rotation axes.

Monogenetic Volcano: One that was formed during a relatively short time period and during a single eruption. Most monogenetic volcanos are basaltic.

Mountain Leather: This term refers to several different asbestiform minerals which occur in a fibrous, flexible, and matted intergrowth which is leather-like in appearance. A few examples of "Mountain Leather," which may also be referred to as "Mountain Cork," "Mountain Wood," or "Leatherstone" are: Actinolite, Palygorskite, Saponite, Sepiolite, and Tremolite.

MYA: This is an abbreviation used to indicate "Million Years Ago."

Native: Occurring in nature, either pure or uncombined with other substances. This term is usually applied to metals.

Natural: (Diamond) A small portion of the original Diamond crystal surface is left unpolished on the girdle of the gemstone. This area, which is called a "natural," indicates that little material was wasted in cutting to obtain a heavier carat weight. Its presence is not desirable on a polished Diamond.

Natural Glass: This is igneous rock that formed during rapid cooling of molten rock. It cooled down too quickly for crystallization to occur. Natural glasses are amorphous with rounded shapes and they usually contain conchoidal fractures. Obsidian is a type of natural glass. Also see Lechatelierite

Negative Crystal: A type of inclusion in which voids, or empty cavities, are bounded by the growing host crystal walls. More simply put, they are a cavity within a crystal. They are in the shape of the original crystal contained within the void/cavity. Small Quartz crystals within a larger Quartz host crystal are a good example of negative crystals. Negative crystals may even contain fluids and/or gases within them.

Nucleus: The center of an atom which is composed of protons and neutrons. It also contains the majority of the atom's mass.

Neutron: A non-charged particle found in the nucleus of an atom.

Nodule: A small rounded lump of a mineral or aggregate, normally without internal structure, and having a contrasting composition from the enclosing sediment or rock in which it is embedded.

Nugget: A lump of native gold, silver, platinum, copper, etc.

Ochre: An amorphous yellow to red substance composed of iron compounds, such as Hematite and Limonite.

Oiling: A process of applying mineral oil to a stone in order to enhance it and mask inclusions, make them more transparent, and darken their color. Emeralds are frequently oiled to mask their many inclusions.

Oily: Poorly reflective luster, similar in appearance to oil.

Oolitic: Having concentric globular deposits smaller than the size of a pea.

Opalescence: A type of iridescence that is most often a light blue colored glow or pearly appearance.

Opaque: Light does not pass through it. It is not transparent or translucent at all.

Orbs: These are round designs of any size or color which resemble eyes. They can be found on many minerals, but they are most commonly seen on Agate and Jasper varieties.

Ore: Rocks or minerals that can be mined for a profit.

Organic Compound ~ Non-Mineral: Substances which are Organic in origin and composition. Their sources are from living organisms. They are amorphous, having no crystalline structure.

Organic Gem: Matter that is not technically a gemstone, but it is derived from animal or plant life. Some organic gems are Amber, Coral, Ivory, Pearl, Jet, and Shells.

Orient: Iridescence in Pearls. It is created through diffraction and interference of the light by the shingle-like layers of Aragonite platelets near the Pearls's surface.

Orthorhombic: (Crystal System) Defined by three unequal mutually perpendicular axes.

Outcrop: The part of a rock formation that appears at the ground's surface.

Outgassing: The release of gases into the atmosphere during volcanic eruptions. A lot of our atmosphere (including water vapor, nitrogen, and argon) came from outgassing.

Oxidation: A chemical process in which oxygen atoms bond to atoms of a material, like a metal, and electrons are transferred from the oxided material to the reduced material. Iron oxidizes when exposed to air and moisture, forming iron oxide (rust). Silver oxidizes (tarnishes) when it is exposed to hydrogen sulfide in the air forming silver sulfide.

Ozone Layer: A region of the stratosphere which contains most of the Earth's atmospheric ozone. It is about 10-25 miles (15-40 km) above the Earth's surface. The ozone layer shields the Earth from harmful ultraviolet rays that come from the Sun. The ozone layer is becoming depleted, and there is an "ozone hole" over Antarctica.

Pacific Ring of Fire: The Pacific "Ring of Fire" is a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. It is shaped like a horseshoe and it is 40,000 km long. It is associated with a nearly continuous series of oceanic trenches, island arcs, and volcanic mountain ranges and/or plate movements. It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt. **For more detailed information and a map, click here: Pacific Ring of Fire

Paleontology: The branch of biology that studies the forms of life that existed in former geologic periods, chiefly by studying fossils.

Pangea ~ Pangaea: The Earth was once a single super-continent named Pangea, or Pangaea. See Continental Drift. **For more detailed information and maps, click here: Pangea ~ Pangaea

Paramorph: This is a pseudomorph involving two minerals with identical compositions, but with different crystal structures. Due to unstable conditions, the original mineral transforms into the other mineral (with the same chemical structure) while retaining the original crystal's shape. Also see Pseudomorph.

Parti-Colored: A mineral specimen or gemstone with multiple distinct colors blended together which are seen at different angles of observation. This is most commonly seen in Tourmaline and Sapphire.

Patina: The change of an object's surface layer that results from aging. Exposure to the air for an extended period of time oxidizes many metals, turning copper and bronze green, and gold reddish. Artificial patinas can be applied to newer objects by using acids or electrolytes.

Pearlescent ~ Nacreous: A material that shows a luminous quality similar to that of the surface of a Pearl.

Pearl Essence: A liquid coating (derived from the scales of herring fish) which adds a pearl-like luster to simulated pearls.

Pegmatite: Coarsely grained igneous rock found usually as dikes associated with a large mass of finer grained plutonic rock.

Perthitic: The separation of one mineral from another by solid precipitation, forming a lattice type intergrowth, usually as thin, roughly parallel, layers in a host crystal.

Petrified: This is an organic substance which has become stone. Organic substances such as shells, stones, and wood embedded in sediments become converted into stone by the gradual replacement of their tissues, particle by particle, with corresponding amounts of infiltrated mineral matter.

Petrology: This is the science of the origin, history, occurrance, structure, chemical composition, and classification of rocks.

Phantom: This is a phenomenon exhibited when a crystal grows and a new growth grows over the old crystal in the same direction. It leaves its inscription on the crystal. Phantoms can be left by whatever mineral washes over the original crystal in the groundwaters passing over it at any time during its growth cycle. Exposure to natural radiation during the crystal’s growth cycle is also known to cause phantoms, especially in the varieties of the Quartz family. The phantoms caused by radiation are usually a dark smoky color. Additional growth may be present, so more than one phantom may be found in a crystal. The phantoms commonly take on the same shape as the crystal itself.

Phenocryst: A relatively large and conspicuous crystal found in a fine grained matrix in porphyritic igneous rock. Volcanic phenocrysts often show flow banding, a parallel arrangement of lath shaped crystals.

Phosphorescence: This may occur after fluorescence. The energy from some shifting electrons (caused by exposure to shortwave and/or longwave ultraviolet lamps) can be "stored" within the material and be released at a later time. If the energy release is delayed for a period of seconds, minutes, hours, or days, it is known as phosphorescence. Or, a mineral which continues to glow for an interval after the ultraviolet light source has been turned off. **For more details on phosphorescence, click here: Fluorescence

Photomicrograph: A photograph taken through a microscope.

Piezoelectric: Capable of producing a surface electric charge when deformed elastically; a property of some minerals without a center of symmetry.

Pillow Lava: This is lava that cools under the sea with an interconnected sack-like form.

Placer Deposit: An alluvial or glacial deposit where heavier minerals (such as gold and platinum) are deposited in the lower layers.

Platy: A mineral habit with flat, thin crystals.

Play-of-Color ~ Color Play: A range of colors seen in a gemstone such as Opal when it is viewed from different angles. The phenomenon is due to optical diffraction interference from small microscopic spheres of Cristobalite included in a silica gel.

Pleochroism: The phenomenon whereby the color intensity or the actual color is different depending on the orientation in which a crystalline substance is observed. The changes in color may be seen from changes in position/angle or lighting conditions. A true color change stone. **For more details on pleochroic and color changing gems and minerals, click here: Pleochroism

Plumose: Having a feathery appearance.

Plutonic Rock: A general name pertaining to igneous rocks formed at great depths. It solidifies as it cools, before it reaches the earth's surface.

Points: Units of measurement to express the carat weight of a gem. One carat is equal to one hundred points.

Polymorphs: These are minerals having the same chemical equivalence of another mineral, but with different atomical arrangements, such as Diamond and Graphite.

Porous: Containing voids, pores, cells, and other openings that may or may not interconnect.

Porphyry: Any of a group of igneous rocks that is characterized by extremely fine grain and which may contain phenocrysts.

Potch: Native material around a gemstone, like Ironstone surrounding Opal.

Precious Gem: A high quality gem that has beauty, durability and rarity. There are presently four official precious gems: Diamond, Emerald, Ruby, and Sapphire.

Precious Metals: They are defined by the industry as Gold, Silver, and Platinum. Palladium (a member of the Platinum family) is sometimes included under this term. **For more details about the precious metals, click here: Precious Metals

Primary Deposit: Where a gem, crystal, or mineral is found residing in its original host rock or matrix.

Prism: A crystal form with three or more similar faces parallel to a single axis.

Prismatic: A crystal habit described as being a crystal with four or more sides similar in length and width. Prismatic crystals are usually elongated in one direction.

Property: One of the physical or chemical characteristics of a material.

Proton: A positively charged particle found in the nucleus of an atom.

Pseudochromatic: Coloring due to physical causes such as dispersion or foreign included particles and internal boundaries.

Pseudomorph: This is a type of mineral formation in which one mineral has chemically replaced another mineral, without changing the external form of the original mineral. They occur in three ways: as paramorphs, infiltration pseudomorphs, and incrustation pseudomorphs. See the indiviual definitions. **For more detailed information on pseudomorphs, please click here: Pseudomorphs

Pulverulent: Easily reduced to powder.

Pumice: An igneous rock which was formed when lava cooled quickly above ground. It is a naturally occuring volcanic glass containing numerous empty gas cavities. It is very light in weight, porous, and resembles foam. It often floats on water.

Pyramidal: A crystal form which has three, four, six, eight, or twelve nonparallel faces that meet at a point.

Pyroclastic Flow: A ground-hugging avalanche of ash, pumice, rock fragments, and volcanic gases, traveling down the side of a volcano at 100 km/hour, or faster. It can have a temperature greater than 500° C. When it comes to a rest, its component solids may compact, deform (flatten) and weld together due to its intense heat and weight.

Pyroelectric: Capable of producing a surface electric charge when temperature changes; a property of some minerals that do not have a center of symmetry.

Pyroxene: This mineral group consists of common, dark-colored, rock-forming silicate minerals. The most common are Diopside and Augite.

Quarry: An open excavation, or pit, from which rock and stone, as opposed to a mineral ore, is obtained by digging, cutting, or blasting.

Quartzite: A hard metamorphic rock which was originally Sandstone. When sandstone is metamorphosed to quartzite, the individual quartz grains recrystallize along with the former, cementing material to form a strong interlocking mosaic of quartz crystals.

Quaternary Period: "The Age of Man," 1.8 million years ago to the present, is the most recent period of geological time.

Radiated: This name is given to crystal aggregates that radiate from a center point without producing star-like forms.

Radiation: Subatomic particles or high energy photons emitted by a radioactive substance, such as Uranium or Thorium.

Radical: Radicals are compounds that act as a single atom when combining with other elements to form minerals. Radicals contain one or more unpaired electrons.

Radioactive: This refers to substances that contain atoms whose nuclei are unstable. This causes the substance to slowly decay and emit radiation.

Radioactivity: A property of minerals that contain radioactive elements. Radioactive elements contain disintegrating nuclei and emit alpha, beta, and gamma rays. Uranium and Thorium are the most well-known radioactive elements. Minerals containing these elements in their structure are known as radioactive minerals.

Rare Earths Elements: Oxides of a series of 15 metallic elements. They are not especially rare in the earth's crust, but economic concentrations are. The rare earth metals resemble one another very closely in chemical and physical properties, so it is difficult to separate them. Lanthanum, Lutetium, Yttrium, and Scandium are a few of them. **For more detailed information, click here: Rare Earth Elements

Reconstructed Stones: This is an old method of fusing stones made from chips and/or other unusable pieces of stone. In other words, fake, not natural.

Refraction: The bending of light (or any wave phenomenon) when it moves between media with different conductive velocities. **For more details about refraction, click here: Refraction

Refractive Index: A mathematical constant equal to the ratio of the velocity of light in a vacuum to that in the substance; it determines the angle at which light bends when it enters a substance obliquely. It is important for gem identification. **For a list of the Refractive Index values of various gemstones, click here: Refractive Indicies Values

Reniform: A term meaning kidney-shaped, which is used to describe rounded mineral surfaces.

Resinous: Having the luster of resin.

Reticulated: This refers to a mineral with cross meshes, like a net.

Reticulite: (Thread-lace Scoria) A very light form of pumice. It is formed during very high fountaining volcanic eruptions, when a lot of air is included in the rock.

Rift ~ Graben: A valley between two faults.

Rock: A consolidated assemblage of grains of one or more minerals. There are three broadly defined groups: igneous, sedimentary, and metamorphic rocks.

Rosette: An intergrowth of numerous platy crystals overlapping like the petals of a rose.

Rough: The raw natural state in which a mineral or gem comes out of the ground. It has not yet been polished, tumbled, or faceted.

Rutile: A titanium dioxide mineral which forms needle-like inclusions within a mineral, crystal, or stone. The needles can be found in many different colors. It is most frequently found in Quartz crystals. Rutile needles can also produce some amazing gem phenomena, such as Stars and Cat's Eyes in Sapphire, Ruby, and many other gems.

Rutilated: Containing Rutile needles.

Sandstone: A sedimentary rock formed by the consolidation and compaction of sand and held together by a natural cement, such as silica. Most sandstone is composed of quartz and/or feldspar.

Sanidinate: A type of igneous rock composed chiefly of Sanidine.

Scaly: Consisting of scales or tabular crystals.

Scepter: A crystal form that is characterized by a sudden increase in diameter at its termination, resembling a royal scepter. The large end of the crystal appears to be sitting on a rod. Or, a convex crystal on a small stem.

Schiller: This is an iridescent or bronze-like luster occuring in some minerals. It is referred to as labradorescence when it occurs in feldspars. This optical effect is caused by submicroscopic lamella contained within the mineral. It can produce a bronze-like luster, golden iridescence, red play of color, and/or a blue-green sheen that flashes when viewed from certain angles.

Schist: A metamorphic rock having a sub-parallel alignment of the principal constituent Mica or Mica-like (platy) minerals.

Scoria: This term refers to bomb-sized (greater than 64 mm across) pyroclastic fragments that are formed as blobs of lava cool when they are blasted through the air. Scoria are filled with cavities formed by trapped air bubbles.

Seamount: An underwater mountain that rises at least 50-100 m above the sea floor. Some seamounts rise above the water's surface. Most seamounts are volcanic in origin; only a few are non-volcanic.

Secondary Deposit: A deposit of gems, minerals, or crystals that has been worn away from its original site, usually by the effects of weather. An example would be an Alluvial Deposit.

Sectile: Capable of being cut with a knife into thin shavings.

Sedimentary Rocks: Rocks that were originally formed from sediment, including shale and sandstone. They are also composed of fragments of other rocks deposited after transportation from their original source. This includes those formed by precipitation or by the secretions of animals, as in certain limestones.

Seismic: A term for vibrations within the earth, both natural and artificial.

Septarium ~ Septarian: An irregular mass of mineral substances (Concretion) having cracks filled in with another mineral, usually calcite.

Series: Any number of rocks, minerals, or fossils having characteristics, such as growth patterns, succession, composition, or occurrence, that make it possible to arrange them in a natural sequence.

Shaft: A vertical, horizontal or inclined passage providing access from the working surface of underground ore bodies.

Shale: A fine-grained clastic sedimentary rock composed of mud, which is a mixture of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite.

Sheen: The way a mineral reflects light. It can be affected by characteristics just below the mineral's surface.

Shield Volcano: A volcano that has gently sloping sides. Shield volcanoes are composed mostly of basalt. The biggest volcanoes on Earth (like those on the islands of Hawaii) are shield volcanoes.

Silica: A dioxide of silicon, which occurs in crystalline form as Quartz, Cristobalite, and amorphous Opal. It is an essential constituent of the silicate group of minerals.

Siliceous Rock: A type of sedimentary rock which has silica as its principal constituent. The most common siliceous rock is Chert.

Silky: Having the luster of silk. It may also refer to silk-like inclusions found in Sapphire and Ruby.

Silt: A particulate material made from loose bits of rock and/or mineral sediment. Silt is finer than sand, but coarser than clay. Silt is carried by moving water. Once it has been deposited it is called sediment.

Simulated Gemstone: Artificial gems of various constructions that look similar to natural gems.

Sixling: A twin intergrowth of six crystals that appears to have hexagonal symmetry; a common habit for Chrysoberyl.

Skarn: A metamorphic rock which usually forms by chemical metasomatism of rocks during metamorphism and in the contact zone of magmatic intrusions, like granites with carbonate-rich rocks such as limestone or dolostone.

Skeletal Crystals: Hollow or imperfectly developed crystals formed by rapid crystallization. Also see Hopper Crystals.

Slag: A glassy substance formed as a by-product from the smelting of metallic ores.

Slate: A fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism.

Smelting: Extracting metal from concentrated ore by pyrometallurgical processes. Reducing agents may or may not be used.

Soluble: Capable of being dissolved in a fluid.

Spalling: This is the chipping, upward, and outward heaving of rock caused by release of pressure.

Specific Gravity: This is the weight ratio of a mineral due to the density of the atomical arrangement and the heaviness of the elements it contains. **For a list of the Specific Gravity values of various minerals and gemstones, click here: Specific Gravity Values

Speleothem: A mineral deposit formed in a cave. Speleothems are made when calcium carbonate or other minerals precipitate (drip out of solution) from slow-moving cave water. Stalactites and stalagmites are examples of speleothems.

Spelunker: A person who enjoys exploring caves.

Spelunking: The hobby or practice of exploring caves

Sphenoidal: Wedge shaped. A tabular habit having the appearance of a book.

Spherules: Rounded masses of acicular crystals, radiating from a central point.

Splintery: The property of certain minerals or rocks to break or fracture into elongated fragments like splinters of wood.

Spongy: A term applied to vesicular rock structures with thin partitions between the vesicles, resembling a sponge.

Spreading Ridge: An area of the ocean floor in which new crust is being formed as magma erupts.

Stalactite: An icicle-shaped mineral deposit, usually of Calcite or Aragonite, which hangs down from the ceiling of a cave. It is formed from the dripping of mineral-rich water that slowly evaporates and deposits some of its dissolved mineral.

Stalactitic: Like a stalactite, a conical or cylindrical mineral deposit that hangs from the ceiling of a cave.

Stalagmite: A mineral deposit usually in the shape of a conical pillar, commonly made of calcite or aragonite. It is built up on the floor of a cave from the dripping of mineral-rich water that slowly evaporates and deposits some of its dissolved mineral.

Stellate: An aggregate of crystals in a star-like arrangement.

Streak: A mineral's powder color. It is most easily observed by rubbing the mineral on a piece of white unglazed porcelain (a streak plate). **For a list of the Streak Colors of various minerals, click here: Streak Colors

Striations: These are grooves, lines, and scratches found naturally in and/or on some minerals.

Subduction Zone: An area on the planet's crust in which the edge of an oceanic continental plate is being pushed beneath another plate.

Subfossil: Remains whose fossilization process is not complete, either for lack of time or because the conditions in which they were buried were not optimal for fossilization.

Sublimate: Changing directly from a solid to a gas, or a gas to a solid, without becoming a liquid. The condensate from such a change is also called a condensate. Water and Sulfur have this property.

Substrate: A thin layer of mineral, often of calcium carbonate, underlying a mineral deposit, and separating it from the host rock.

Subvitreous: Luster not as highly reflective as adamantine, but more so than vitreous.

Supervolcano: An enormous volcano that is an order of magnitude larger than ordinary volcanoes. A supervolcano occurs when a huge magma chamber in the Earth's crust erupts after being under great pressure, causing a large caldera to form as the land over the magma chamber collaspses. This type of eruption is probably able to cause a major climate change and a mass extinction as it ejects dust, debris, and caustic gases into the atmosphere. Supervolcanos do not look like the familiar cone-shaped volcano. A supervolcano looks like a huge collapsed crater. Yellowstone National Park is a supervolcano. It is due to erupt soon (in terms of geological time).

Symmetry: The correspondence in shape or length of elements of a body; as repeated by a mirror, rotation about an axis, or inversion through a point (center of symmetry).

Synthetic Gemstone: A man-made stone that, unlike a simulant, has the same chemical composition and crystal structure as its natural counterpart.

Tabular: A crystal habit with the appearance of a paper tablet.

Tailings: The fine grained remnant of ore that has had the economically extractable valuable material removed from it by the concentration process.

Talisman: A good luck charm. Also see Amulet

Tectonic Activity: The shifting of the planet's surface because of changes deep inside the body. Earthquakes, fissures, rifts, and volcanoes are some results of tectonic activity.

Tektite: (Meteoric Natural Glass) This is a natural glass formed from meteorite or other cosmic impacts melting the local rock. Depending on the nature of the latter, the composition of tektites is variable. Many varietal names are given to tektites from different localities. These are just a few of the named tektites and their place of origin: Bediasite and Georgiaite are found North America, Moldavite is found in Central Europe, Ivorite is found in West Africa, and Australian and Asian tektites are known as Indochinite, Australite, Zhamanshinite, Irgizite, and Darwin Glass. Some feel that it is still unclear if they originated as molten material from meteorites, or as terrestrial rock melted by the meteorite's impact on Earth. Also see Lechatelierite

Tenacity: The strength of a mineral. It is also its resistance to breaking, crushing, bending, or tearing.

Tenebrescence: It is also known as reversible photochromism. It is the ability of certain minerals to change color when exposed to sunlight and other types of illumination, such as ultraviolet lighting. The effects can be repeated indefinitely, but will be destroyed by heating. It is most commonly seen in the minerals Tugtupite and the Hackmanite variety of Sodalite.

Tephra: Volcanic material that is airborne, like lava bombs, lapilli, and ash.

Termination: The end of a crystal that is much longer than it is wide or thick.

Tetragonal: (Crystal System) Defined by three mutually perpendicular axes, two of which are of equal length.

Thermoluminescence: A phenomenon exhibited in certain minerals in which they give off light, like a glow, when heated. This can only be observed in a dark area.

Trachyte: A lightly colored and very finely grained extrusive igneous rock which is chiefly composed of alkali feldspar, along with minor amounts of dark colored minerals, such as biotite, amphibole, or pyroxene.

Transformation: This occurs when minerals transform into other minerals under stress, heat, prolonged exposure to light, or upon loss of water.

Transparency: The degree to which light passes through a material.

Trap Rock: A type of igneous rock. This solidified lava often contains pockets of crystals.

Triboelectric: The property of certain crystals of generating an electric charge when in contact with another material.

Triboluminescence: A property exhibited by certain minerals which causes them to give off orange or yellow flashes of light when they are sawed or struck together. It is best observed in a dark area.

Trichroism: Gems that display three different colors when viewed from different angles. **For more details on pleochroic and color changing gems and minerals, click here: Pleochroism

Tri-Colored: A mineral specimen or gemstone with three distinct colors which are clearly separated from each other. This is most commonly seen in Tourmaline.

Trigonal: (Crystal System) Defined by three equal axes lying in a plane and intersecting at 120 degree angles and a fourth perpendicular axis that is a three-fold rotation axis.

Trilling: This is a twin intergrowth of three crystals that appear to have trigonal symmetry.

Triplet: A man-made stone of three parts that includes a clear protective top layer fused together with a thinly sliced gem, a dark backing material, and a clear or colored adhesive. Most commonly found with Opals.

Troposphere: The lowest region in the Earth's atmosphere. On the Earth, it goes from ground or water level up to about 11 miles (17 kilometers) high. The weather and clouds occur in the troposphere. In the troposphere, the temperature generally decreases as altitude increases.

Tufa: A terrestrial sedimentary rock which is formed by the precipitation of carbonate minerals from ambient temperature bodies of water. This is not to be confused with tuff, which is listed below.

Tuff: This is a porous igneous rock composed of volcanic ash compacted together.

Tufted: This is a crystal aggregate in the form of fibrous crystal clumps.

Tumbling: This is the simplest form of gem polishing, in which the rough mineral is put into a revolving barrel with progressively finer abrasives until a fine polish is obtained. It is a very time consuming project. It may also be done by hand in some cases.

Tuya: A volcano which erupts under a glacier.

Twin: (Twinned Crystal) A nonparallel intergrowth of separate crystals related by symmetry not possessed by the substance. Or, two intergrown crystals that share a common internal or external axis or plane.

Type Locale: The source location of a mineral specimen first used to determine and describe a mineral's physical, optical and chemical properties.

Ultramafic Rocks: A type of igneous rock composed mostly of the mafic minerals: hypersthene, augite, and/or olivine.

Ultraviolet Light: Electromagnetic radiation that is not readily visible, having the wavelength of from 4 nanometers to 400 nanometers (just beyond the violet in the visible spectrum). There are four bands of ultraviolet light: Long Wavelength, Medium Wavelength, Short Wavelength, and Vacuum Ultraviolet Light. Shortwave and Longwave UV lighting reactions are readily used as diagnostic tools in identifying some minerals. **For more details about UV light, click here: Ultraviolet Light

Unit Cell: The unit cell is the smallest divisible unit of a mineral that still possesses the atomic symmetry and physical properties of the mineral.

Valence: This is the number of electrons that an atom will lose, add, or share when it reacts with other atoms.

Variety: (Gemstone) A named specific color or other quality of a gemstone species, such as Ruby for red corundum.

Vein: A zone or belt of mineralized rock lying within boundaries clearly separating it from neighboring rock. It includes all deposits of mineral matter found through a mineralized zone or belt coming from the same source, impressed by the same forms, and appearing to have been created by the same method.

Veinlet: A small vein.

Vent: An opening in the Earth's surface through which volcanic materials erupt.

Vesicle: A small cavity in a glassy, igneous rock formed by the expansion of a gas bubble or steam during the solidification of the rock.

Viscosity: A measure of the resistance to the flow of a liquid. Viscous liquids flow very slowly; less viscous fluids flow more quickly. For example, water has a low viscosity, while oil has a higher viscosity.

Vitreous: Luster similar to freshly broken glass; brightly reflective.

Volatile Elements: They have very low boiling temperatures. Examples include nitrogen, helium, water, and carbon dioxide. Volatiles are usually found in liquid or gaseous form, except at very cold temperatures.

Volatiles: (Components) In magma, those materials that readily form a gas and are the last to enter into and crystallize as minerals during solidification.

Volcanic Rock: A type of igneous rock that was originally lava, or magma. It reached the surface of the earth before it hardened.

Volcano: An opening in the earth's crust through which molten lava, ash, and gases are ejected.

Volume: The amount of space which is occupied by a mass.

Vug: An open cavity in a rock which is often lined with tiny crystals.

Weathering: This is the process of chemical and mechanical disintegration of rocks and minerals by exposure to the atmosphere, sun, rain, heat, cold, oxidation, and reactions with other environmental agents.

White Light: This is visible light that has all of the visible spectrum present, so it appears white.

Xenolith: Rock which is enclosed in volcanic magma.

Zeolite: Zeolites are a group of microporous aluminosilicate minerals that are related in structure, habits, and occurrence. They are usually found in zeolite deposits which are of igneous origin. **For more detailed information on Zeolites, please click here: Zeolites

**More coming soon.

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