Glossary - America's Best Rockhound Glossary

Letters - A

Abrasive – Substance used to cut and polish rocks.  Often (incorrectly) called ‘grit.’

Adit – A mine entryway; usually horizontal.

Agate – Agate is a type of microcrystalline quartz called chalcedony or cryptocrystalline quartz.  This means that individual crystals are to small to see with your eye.  Agate is the most common variety of chalcedony.  Agate is, in fact, identical with quartz in composition and physical properties.  There are many, many varieties of agate (e.g., banded, fortification, moss, picture, plume, etc.), many of which have been used as gems for centuries.  Agate usually forms in cavities in volcanic rocks.

Agate Probe – A metal rod with a handle used to detect underground agates (as well as jasper and chert). 

Amber – A gem made from hardened resin of ancient trees.  After the resin (sap) was buried in the ground, it turned into a mineral.  Valuable as a jewelry/decoration.

Amethyst – A pale purple to violet variety of crystalline quartz.  The color is due to iron compounds.

Amorphous – Non-crystalline.  A mineral or rock that lacks crystal structure; a formless mass.

Amygdaloidal Rock – Volcanic rock containing numerous gas cavities (amygdules) filled with such secondary minerals as quartz, calcite, chalcedony, or zeolite.

Amygdule – A mineral-filled cavity or vesicle in a volcanic rock.  Cooling of the fluid rock around a gas bubble forms the cavity.

Arête – A narrow mountain ridge formed between two glacial valleys or cirques.

Asterism – The property of reflecting light in star-like patterns of 4, 6, or 12 rays.  Examples include:  ruby, sapphire.


Letters - B

Basalt – An igneous rock.  The most common type of volcanic rock, formed as lava cools and hardens.

Batholith – A very large mass of igneous rock (usually granite) that formed deep underground and is later exposed by erosion.

Bauxite – The rock ore from which aluminum is refined.

Bedding – Layering of sedimentary rocks.

Bedrock – The layer of solid rock underneath loose material such as soil or sand.

Biogenic Rock – Rock formed by organic action.

Birefringence – An optical effect commonly known as double refraction.  Calcite (especially Iceland spar variety) exhibits double refraction.    

BLM – Bureau of Land Management.  The BLM manages hundreds of millions of acres of federal public lands, primarily in the Western United States.

Botryoidal – Formed into masses of small spherical bulges; often in grape-like clusters.  Used, for example, to describe hematite and some agate textures.

Breccia – Sedimentary rock made of jagged or angular mineral fragments naturally cemented together.


Letters - C

Cabochon – A rock that has been cut in such a manner that its top has a smooth domed surface with no facets.  Oftentimes oval or round in shape.

Calcareous – Composed of calcite or calcium carbonate.

Calcite – The main mineral in limestone and chalk.  Its chemical name is calcium carbonate.

Canyon – A deep, narrow valley often formed when a river cuts through rock.

Carat – The standard measure of weight for precious gems and minerals.  One carat is equivalent to 0.006 ounces (0.2 grams).

Carboniferous – Geological time period that stretches from around 365 – 290 million years ago.

Cast – A fossil formed by the infilling of a mold.  For example, ‘wood casts’ may form where trees or parts are buried, then disintegrate, and agate fills the void.

Cat’s-eye effect – See chatoyancy.

Cave – A naturally formed opening in the ground that extends beyond the zone of light and is large enough for a person to enter.

Cavern – A large chamber within a cave.

Chatoyancy – An optical effect popularly known as ‘cat’s-eye effect’ in which certain gemstones, when polished, will reflect bright light as a single, thin ray (like a cat’s eye pupil).  Chatoyancy is caused by fibrous inclusion that run parallel to one another.  Rounding the stone (a polished cabochon or sphere) concentrates the light and increases the effect.  Polished tiger’s eye quartz, for example, can exhibit chatoyancy.

Chemical Sedimentary Rock – Rock that is formed by mineral precipitation.

Chitin – The horny substance that forms the shell of arthropods.

Cinder Cone – The naturally expelled debris, usually in a conical pile, surrounding the vent of a volcano.

Cirque – A steep-sided, bowl-shaped hollow in a mountain, carved out by a glacier.

Claim – A tract of public land lawfully staked out to mine.

Clast – Fragment of broken rock.

Clastic Sediment – Sedimentary rock made mainly of broken rock fragments.

Clay – Mineral particles that are smaller than 0.00008 inches (0.002 mm) wide, and are common in soil and sediment.  Clay forms by the weathering of granite and other rocks.

Cleavage – The way certain minerals break along certain planes according to the mineral’s atomic structure.

Columnar – Like columns.  For example, columnar basalt.

Compound – Two or more elements joined together chemically.

Compression Fossil – A fossil preserved in sedimentary rock that has undergone physical compression.  Carbonized leaves are examples of compression fossils.    

Concentrates – Material that has been processed (e.g., washed, screened) and therefore reduced typically by removing large rocks and organic material.

Conchoidal – Term used to describe a concave (curved or shell-like) fracture in a glassy mineral.  For example, obsidian will make a conchoidal fracture.

Concretion – A distinct, hard, compact mass or aggregate of mineral matter that normally is rounded but may be very odd shaped.  They usually are formed by chemical precipitation.

Conflict Minerals – Minerals (especially diamonds and other gemstones) that are mined in war zones to finance terrorist or revolutionary groups.    

Conglomerate – Sedimentary rock made up of pieces of rocks that have been worn into rounded shapes (essentially the same size) that have been naturally cemented together.  Also known as puddingstone.

Conodonts – Teeth of very early fish-like creatures that lived about 515 million years to 210 million years ago.

Contact Metamorphic Rock – Rock that forms as the heat from magma or a lava flow alters an existing rock.

Coprolite – Fossilized dung.

Coral – This gem comes from the skeletons of tiny sea animals that live in colonies.  A coral polyp takes up calcium carbonate minerals from the sea and builds itself a hard limestone home.  The most valuable coral is red.

Core – The center of the earth, made of heavy metals.  It is about 850 miles across.

Country Rock – The rock that surrounds a mineral deposit or igneous intrusion.

Crater – The natural depression marking the vent of a volcano or impact site of a meteorite.

Craton – Ancient part of a continent unaltered for at least one billion years.

Cretaceous – Geological time period that stretches from about 145 – 65 million years ago.

Crinoids – A large class of echinoderms usually having a somewhat cup-shaped body with five or more feathery arms.

Crust – The thin outer rocky layer of the earth that lies over the mantle.  The crust is between four and forty-four miles thick.  There are two main types of crust:  continental and oceanic.  Continental crust is thicker than oceanic crust.

Cryptocrystalline – Describing a rock surface or solid rock that is comprised of extremely minute or microscopic crystals.

Crystal – The special shape (geometric form) in which many minerals form.  A crystal has fixed properties and the sides usually are flat and regular.  For example, a quartz crystal has six sides.

Crystal Form – The way in which the different faces of the crystals are arranged.

Crystal Habit – The typical shape in which a crystal or cluster of crystals grows.

Crystal System – One of the different groups into which crystals may be placed according to how they are symmetrical.


Letters - D

Dana Number – Number assigned to each mineral according to the classification system devised by James Dwight Dana.

Dendrites – Fern-like growths of crystals found lining the cracks and joints in rocks.

Deposit – To lay down a material such as mud, sand, or gravel in a new location.

Desert – An arid region that typically receives less than ten inches of rain a year.

Desert Rose – Nickname for rosette-shaped gypsum concretions and barite formations.    

Devonian – Geological time period that stretches from about 415 – 360 million years ago.

Diatom – Fossilized microscopic marine plant.

Dichroic – Showing different colors when viewed from different directions.  Certain forms of tourmaline are dichroic.    

Dike (sometimes Dyke) – A sheet-like igneous intrusion that is near vertical or cuts across existing rock.

Dinosaurs – The group of now extinct prehistoric reptiles that lived between 230 and 65 million years ago.  Their closest living relatives include crocodiles and birds.  Yep, birds.

Dip – The downward angle of inclination of bedrock from the horizontal.

Doublet – A composite stone made of two pieces cemented or glued together - e.g., opal doublets.

Dredge – Mining equipment used to find gold, sapphires, or garnets.  Typically, dredges are used in streams to suction up material from the streambed to process through a riffle or other device to screen out heavy material (e.g., gold).

Drusy – A crust of uniformly tiny crystals filling a cavity or coating a rock.

Dumpsite – At a mine site, where the non-economic rocks are deposited.  Rockhounders often will search dumpsites as these piles will contain material that took enormous effort to bring to the surface and may contain interesting specimens.


Letters - E

Electrum – A natural mixture of gold and silver.

Element – Material that cannot be broken down into more simple substances by chemical means.

  The simplest, most basic substances – such as gold – each with its own unique atom.

Enhydro – A crystal that contains a fluid-filled cavity.

Eocene – Relating to, or being an epoch of the Tertiary Period between the Paleocene and the Oligocene or the corresponding system of rocks. About 55 to 35 million years before the present.

Epoch – Division of geological time, part of a period.

Era – Largest division of geological time lasting hundreds of millions of years.

Erosion – The wearing away (eroding) and removal of exposed material by water, wind, and ice.

Erratic – A large rock that has been carried by a glacier and dropped from its place of origin.  We have a couple of large erratics in our yard (one is about half the size of a car).

Erosion – The wearing away of the material on the Earth’s surface.

Eruption – A discharge of lava, pyroclastic material, or gas from a volcanic cone or vent.

Evaporite – A natural salt or mineral formed by the process of evaporation (gypsum, halite).

Exfoliation – The process of weathered rock sheets splitting away in layers from larger rock masses.

Exposure – Where a rock outcrop is exposed at the surface.

Extrusive Igneous Rock – Igneous rock that forms at the earth's surface.  The type of rock that forms when volcanic lava cools and solidifies.


Letters - F

Facet – Flat surface on a cut gemstone.

Fairy Cross – Twin crystal of staurolite.

Fault – A long fracture in rock along which rock masses move.

Feldspar – Feldspars are the most common, rock-forming minerals and constitute 60 percent of the earth’s crust.  They include gem varieties such as labradorite and they weather over time to produce clays.

Feldspathic – Rock containing feldspar.

Felsic – Rock rich in feldspar and silica, typically light in color.

Fire Agate – Fire agate is a form of chalcedony that contains inclusion of iron oxides that result in a play of colors much like that of precious opal.

Fissure Volcano – Volcano that erupts through a long crack.

Float – Pieces of rock that have moved from their original source.

Flood Basalt – Plateau formed from huge eruption of basalt from fissure.

Flowstone – The general term for the sheet-like mineral deposit on a cave wall or floor.

Fluorescence – The emission of brightly colored light by a substance when exposed to ultraviolet light rays.

Foliation – Flat layers of minerals in metamorphic rock formed as minerals recrystallize under pressure.

Fool’s Gold – Pyrite.  Named so to emphasize that pyrite is not gold.

Formation – A related deposit of rocks formed during the same time period and under generally the same circumstances.

Fortification – A form of agate with angled concentric banding.

Fossil – The remains, traces, or impressions of organisms (plants and animals) that lived millions of years ago and usually are preserved in rocks.  Some of the oldest fossils are trilobites which can be up to 500 million years old which is much older than dinosaurs.

Fossil Fuels – The fuels coal, oil, and natural gas all of which are formed from the remains of once-living organisms.

Fracture – The way in which a rock or mineral breaks when it does not break along cleavage planes.

Fulgurite – Sand fused by lightning.

Fumarole – A hole or vent through which a volcanic gases escape.


Letters - G

Gangue – Waste rock in a mineral deposit that is being worked.

Gastroliths – Stones, swallowed by dinosaurs and other animals, used to help grind up food.

Gemstone – A mineral, usually crystalline, that derives particular value from its beauty, color, rarity, and hardness (e.g., diamond, ruby, sapphire, emerald, etc.).

Geode – A rock containing a crystal-lined cavity in which the crystals grow toward the center.

  Typically, a portion of the geode is hollow, but lined with quartz.  Geodes occur in lava or limestone.

Geologic Time Chart – Record of divisions of the Earth’s history.

Geological Column – Diagram depicting the successive layers of strata that have formed over geologic time, with the oldest at the bottom and the youngest at the top.  Grand Canyon, Zion, and Bryce Canyon National Parks use geological columns to explain the formations and the relationships between the Parks.

Geologist – Someone who studies the earth.  Often times, geologists study rocks and fossils to learn more about the earth’s history.

Glacier – A large mass of ice that forms on land and flows slowly downhill under its own weight.

Gold Dore – A bar of gold that usually is about ninety percent pure but also includes other metals.  These bars are subsequently smelted and treated to remove the unwanted metals to achieve 0.999 fine gold.

Grain – A particle of a mineral.  Rocks can be described as fine grained or coarse grained depending on the size of the particle.

Graphite – A mineral made entirely of the element of carbon.  Graphite is very soft and black.

Grubstake – Supplies furnished to a prospector in return for a share in discoveries.  A good reason to take an extra candy bar when rockhounding.


Letters - H

Habit – The general shape or appearance of a mineral.

Hackly Fracture – Jagged breakage surface of a metal.

Hardness – Degree of resistance of a rock or mineral surface to being scratched.  The relative hardness Mohs scale uses a 1-10 scale.

Horn – A steep-sided mountain peak, formed by erosion by several glaciers.

Hydrothermal – Related to water heated by magma.

Hydrothermal Deposit – Mineral deposit formed from mineral-rich hydrothermal fluids.


Letters - I

Ice Age – For rockhounds, the Ice Age usually refers to the most recent ice age - a portion of the Pleistocene epoch that began about 1.8 million years ago and ended about 11,000 years ago.  During this time, the Earth went through a series of cold climates in which much of North America was covered by glaciers.

Ichnofossils – Ichnofossils (or trace fossils) are typically sedimentary structures consisting of a fossilized track, trail, burrow, or tube resulting from the life activities and behavior of an animal, such as a mark made by an invertebrate creeping, feeding, hiding, or resting on or in soft sediment.  Some non-sedimentary examples include tooth marks (resulting from predation), skin impressions, and coprolites (fossil dung).

Ichnologist – Scientist who studies fossilized footprints, tracks, and other trace fossils.

Igneous Rock – A type of rock that forms as lava or magma cools and hardens.  Extrusive igneous rock forms on the earth's surface from lava.  Intrusive igneous rock forms underground, in the earth's crust, from magma.

Impact Crater – Crater formed by the impact of a meteorite.

Inclusion – The presence of a foreign body within the a crystal (example – rutile in quartz crystal).

Index Fossil – A key fossil used to correlate strata.

Intrusion – Emplacement of magma into existing rock.

Intrusive Igneous Rock – Igneous rock that forms beneath the surface of the earth.

Invertebrates – Animals lacking a backbone or spinal column (invertebrate fossils are regulated differently than vertebrate fossils).

Iridescence – The play of colors on the surface of a mineral like a film of oil on water.

Island Arc – Curved chain of volcanic islands in a subduction zone (Alaska’s Aleutian Islands).


Letters - J

Japan Law Twin – Twin quartz crystals joined in the same plane at an angle of about 84 degrees.

Jet – A form of coal that can be polished to look like black glass.

Jurassic – Geological time period that stretches from about 200 – 145 million years ago.


Letters - K

K-T Extinction (K-T Boundary Event) – Approximately 65 million years ago, at the end of the Cretaceous and just before the beginning of the Tertiary, the fossil record suddenly changes.  At this time, approximately three-quarters of all life forms go extinct.  The “K” refers to Kreide, meaning chalk in German and references the chalky sediment layer from that time.  The “T” refers to Tertiary.

Karat – Parts per 24 of pure gold in an alloy.

Karst Topography – Geologic formation shaped by the dissolution of a layer(s) of soluble bedrock.  The landscape is characterized by the presence of sinkholes, caves, springs, and underground streams.

Kimberlite – Igneous rocks rich in volatiles, normally forming pipes.

Knapping – The process of chipping away at a stone (such as flint, agate, chert, obsidian) to produce projective points, knives, and other objects.    


Letters - L

Laccolith – A mass of intrusive igneous rock with a dome-shaped top and generally flat base.

Lapidary – A craftsperson who cuts and polishes gemstones.

Last Chance Gulch – Location of significant gold rush in Helena, Montana.

Lava – The hot melted rock that flows out of an erupting volcano.  Inside the volcano, the melted rock is called magma.

Lava Tube – A hollow formation created not by erosion but rather when molten lava solidifies on the exterior but the insulation allows the interior to continue to flow and empty creating a hollow tube.

Leaverite – Rockerhounder slang for a rock that isn't worth carrying back home; just leave 'er right there.

Lithophysae – Scientific name for a thunderegg (Latin for ‘rock bubbles’).

Lithosphere – That portion of the Earth comprising the crust and upper-most portion of the solid mantle.

Locate – To mark the boundaries of a claim.

Lodestone – A piece of magnetite, a naturally occurring magnetic iron-oxide.

Loupe – A small hand-held lens.

Luster – The way a mineral shines caused by reflected light.  Various terms such as metallic or resinous, bright or dull may be used to describe the luster of a mineral.


Letters - M

Mafic – Rocks rich in magnesium and ferric (iron) compounds.

Magma – The hot, molten rock deep in the earth's crust that becomes lava on the surface.

Magma Chamber – Underground reservoir of magma beneath a volcano.

Mantle – Layer of the earth between the outer core and the crust.  It is approximately 1,800 miles thick.  The mantle is made of solid rock that is very hot and slightly viscous, allowing it to flow and circulate over long periods of time.

Massive – Having no definite crystal form or structure.

Matrix – The mass of rock in which larger crystals or fossils are embedded.  Also known as groundmass.

Metamorphic Rock – A type of rock that has been changed by intense heat and/or pressure causing physical and chemical changes as well as the partial melting and recrystallizing of its minerals.

Metamorphism – Process by which rocks and minerals are transformed by heat and pressure to create a new rock.

Meteor – A rock from space that burns up as it enters the earth's atmosphere.

Meteorite – A piece of rock from space that hits the earth's surface without completely burning up.

Microcrystalline – A mineral structure in which the crystals are too small to be seen with the naked eye.

  You need a microscope to see the crystals.

Micrometeorite – A microscopic meteorite.

Micromount – Small mounted rock or mineral.

Mineral – A naturally occurring, inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal structure, and physical properties.  Minerals are the building blocks of rocks.  Accordingly, all rocks are made from minerals.

Mineral Vein – Cracks in rocks that become filled with hot, mineral-rich liquids.

Mineralogist – A scientist who specializes in the study of minerals.

Mineralogy – The study of rock forming minerals.

Mohs' Scale – A scale devised by the Austrian mineralogist Friedrich Mohs that measures the relative hardness of minerals.

Mold – The fossilized imprint of the body of an organism.

Moqui Marble – A dark brown concretion that weathers out of the Navajo Sandstone Formation in southern Utah.  The concretions form when groundwater containing iron flowed through the formation and precipitated out as iron oxides.

Moraine – An accumulation of debris deposited by the outermost edge of a glacier.  Our yard.  Seriously.  A two hundred foot deep gravel deposit from the end of the last ice age.

Mother Lode – The principal vein or lode; the richest and largest source of a desired mineral.

Mudstone – Sedimentary rock composed of fine-grained particles.


Letters - N

Nacre – Tiny platelets of calcium carbonate that create the soft sheen on pearls and inside some sea shells as they reflect light.

Native Element – Element that occurs naturally uncombined with any other element.

Nodule – A rounded lump of mineral found in sedimentary rock. 


Letters - O

Ocean Spreading – Process in which oceans widen as new rock is brought up at the mid-ocean ridges.

Oolith – Small, rounded grains (usually calcareous) that make up some sedimentary rocks.

Oolitic – Made mostly of ooliths.

Opaque – Describing a 

rock or mineral through which light does not pass.

Orbicular – Containing solidly encased orbs or spherules with eye-like patterns.

Ore – Natural material from which useful metals may be extracted.  For example, gold ore; copper ore.

Outcrop – A geologist’s term for the area in which rocks are exposed at the earth’s surface.

Oxidation Zone – Upper layer of mineral deposit where minerals are altered by oxygen and acids in water.  This process also is why dad has to power-wash the large glacial erratics in our yard so that they look nice.


Letters - P

Paleoecology – The study of the ecology of fossil animals and plants.

Paleontology – A science dealing with the life of past geological periods as known from fossil remains.

Pegmatite – Very coarse-grained igneous rock similar to granite in chemistry but characterized by very large mineral crystals, usually found in veins and pockets around large plutons and rich in rare minerals.  Some pegmatites have feldspar, quartz, or other mineral crystals of extraordinary size.

Penguin Motto – No one swims alone.  Smart rockhounders follow the Penguin Motto and go rockhounding with a buddy.  That way, in the event of injury or a problem, there is help.

Period – A division of geological time lasting tens of millions of years.

Permian – Geological time period that stretches from about 300 – 250 million years ago.

Petrified – Turned to stone.  For example, petrified wood or petrified dung (also called coprolite).

Petrologist – A geologist who specializes in the study of rocks.

Petrology – A branch of geology that deals with the origin, occurrence, structure, and history of rocks.

Phaneritic – Grains visible with the naked eye.

Phantom – Earlier growth enclosed within a crystal.

Phenocryst – A relatively large crystal in igneous rock.

Piezoelectricity – Electricity produced in a crystal by pressure.

Pillow Basalt – Lava formed on the seabed consisting of pillow-shaped formations created when molten material contacted water.  Crystals often occur in the cavities of pillow lava.

Placer – Natural deposit of valuable minerals such as gold washed into loose sediments such as river gravels.  Gold found in its natural state without any host rock.  Placer gold (dust or nuggets) typically is found in streams (current or historic) where it has washed away from its source.

Plate – A large fragment of the earth's crust.  Also known as a tectonic plate.

Plug – The solidified core of an extinct volcano.

Pluton – Any large mass of igneous rock that formed beneath the Earth’s surface by solidification of magma.

Polymorphs – Different minerals created by different crystal structures of the same chemical compound.

Porphyritic – Igneous rock containing lots of phenocrysts.

Properties – The characteristics of rocks or minerals by which they may be identified.

Pry Bar – A steel tool used to move heavy rock or expand a crack.  A ‘crow bar’ is a type of pry bar.

Psammology – The branch of geology relating to the study of sand.

Pseudofossil – A natural object, structure, or mineral that resembles a fossil, but actually is not a fossil.  Common pseudofossils include manganese dendrites that form tree-like or fern-like patterns.  Other common pseudofossils include flint and chert nodules, pyrite ‘sun dollars,’ and some concretions.

Pseudomorph – A crystal with the outward form of a different mineral species.  The term also is used to describe other substitutions such as petrified wood where quartz becomes a pseudomorph of wood.

Pumice – A bubbly lava that has solidified in a light, frothy state (sometimes light enough to float on water).

Pyroclast – Fragment of solid magma plug ejected during volcanic eruption.


Letters - Q

Quarry – Often a steep-sided open excavation worked to remove rock.

Quartz – Quartz is the most common mineral on the face of the earth.

Quartzite – A conglomerate or sandstone that is composed primarily of coarse quartz gains or pebbles converted to solid rock through metamorphic action.


Letters - R

Recrystallization – The formation of new mineral grains in a rock while the rock is in a solid state.

Replacement – Substitution by mineral matter.

Riker Mount – Paperbound, glass covered box for displaying specimens.

Rock – A mass of mineral material.  Rocks are divided into three groups:  igneous (fire rocks), sedimentary (cement rocks), and metamorphic (changed rocks).

Rock Cycle – The continuous cycle through which old rocks are transformed into new ones.  For example, an igneous rock may erode and the material becomes a sedimentary rock that may then become a metamorphic rock that may then erode and so on.

Rock Saw – Generally speaking, there are two types of rock saws:  slab saws and trim saws.  The primary difference is size.  Generally, a trim saw has a blade that is four to ten inches in diameter whereas a slab saw has a blade that is ten inches or more in diameter.  Rock saws use circular disk blades with diamond rims.

Rough – Term used to describe uncut or unpolished rock.  Natural rock.


Letters - S

Sandstone – Sedimentary rock mostly composed of sand-sized quartz grains.

Scepter – For mineral collectors, a scepter is a large crystal capping a slender crystal stem.  Scepters result from an interruption of crystal growth and are a rare type of crystal formation.    

Schiller Effect – The brilliant play of bright colors in a crystal, often due to minute, rod-like inclusions.

Scree – A mass of loose rubble and gravel on a mountain slope caused by weathering.

Secondary Mineral – A mineral that replaces another mineral as the result of a weathering or alteration process.

Sediment – Solid grains that have settled out of water.

Sedimentary Rock – A type of rock that originates on the Earth’s surface and forms when sediment is compressed and cemented.  It consists of layers of rock fragments or other substances (e.g., sea shells) that have been deposited on top of each other.

Seismology – Study of earthquakes.

Shaft – A vertical mining excavation.

Silica – Silicon dioxide, a tremendously abundant mineral that occurs widely and in manner forms including, quartz, opal, jasper, and chert.

Sill – Sheet-like igneous intrusion either near horizontal or following existing rock.

Smelting – The process by which a metal is removed from an ore.

Specific Gravity – The ratio of the mass of the mineral to the mass of an equal volume of water.  Specific gravity is numerically equivalent to density (mass divided by volume) in grams per cubic centimeter.

Speleogenesis – The formation and development of caves.

Speleology – The scientific study of caves.

Speleothems – The secondary mineral deposits formed in caves.  These cave formations include cave popcorn, columns, draperies, flowstone, helictites, rimstone dams, soda straws, stalactites, and stalagmites.

Stalactite – An icicle-shaped mineral deposit hanging from the roof of a cavern or cave.

Stalagmite – An upside down icicle-shaped mineral deposit that rises from the floor of a cavern or cave.

Stope – The series of ‘steps’ above or below a mine level where ore has been extracted.

Strata – A sequence of rock formations that can informs us of geological processes over time.

Stratification – Bedding in sedimentary rock.

Streak – The color that a powdered mineral makes when rubbed across an unglazed tile.  The streak is a useful clue to identify a mineral.

Strewn Field – The geographic area on the surface of the Earth where meteorites from a single fall are dispersed.

Striation – A minute parallel groove or line on a crystal face.  Pyrite, for example, may display an obvious striation.

Stromatolite – A rocklike mound produced by microorganisms growing in shallow water.  Fossilized stromatolites are among the oldest signs of life on Earth.

Subduction Zone – Boundary between two tectonic plates where one plate descends into the mantle beneath the other.


Letters - T

Talus – A sloping heap of naturally fallen debris at the base of a mountain or cliff.

Tectonic – The geological processes within the earth that affect the earth's crust.

Tectonic Plate – One of the approximately two-dozen giant slabs into which the Earth’s rigid surface is split.

Tektite – Glassy rock formed by a meteorite impact.    

Thunderegg – A volcanic nodule that typically features a rhyolitic outer shell and a central core of agate, opal, and/or crystals.  The scientific name for a thunderegg is lithophysae (Latin for ‘rock bubbles’).

Topographic Map – A map of an area that depicts mountains and valleys by using ‘topographic contour lines.’  It also usually contains information on roads, rivers, towns, and other features you may come across in a given area.

Topography – The general shape of the surface of the earth, particularly used to define to the shape of the hills and valleys.

Translucent – Description of a material that light is able to pass through, but that is not clear enough to look through.

Transparent – Clear enough to look through.

Trichroic – Showing three different colors when viewed from different directions.  Natural tanzanite is trichroic.  

Trilobites – A group of prehistoric arthropods named after the three lengthways lobes of their bodies.  Trilobites lived in the sea and survived for over 250 million years.

TV Stone – See Ulexite.  

Twin Law – Law stating the way in which the parts of a twin crystal are related.

Twinned Crystals – Two or more crystals of the same mineral that intersect each other along a common, or shared, plane.


Letters - U

Ulexite – A borate mineral composed of long, thin crystals that often occur as a blocky mass of fibrous veins.  When cut and polished (perpendicular to the crystal fibers), and placed on top of a page of text, the text will appear at the top of the specimen.  The individual crystals that make up the specimen will transmit light from the bottom surface of the specimen to the top surface, which resulted in the popular term ‘TV Stone.’

Ultramafic Rock – Igneous rock with a silica content of less than 45%.  Also known as ultrabasic rock.

Ultraviolet Light – A type of light that is long or short wave that is used to identify rocks that fluoresce (or glow) in a variety of colors.


Letters - V

Vein – A thin, sheet-like fracture in a rock that is filled with secondary minerals such as quartz or calcite.

Vertebrates – Animals with a backbone or spinal column. Dinosaurs were vertebrates.  Vertebrate fossils are regulated differently than invertebrate fossils.

Vesicle – Small cavity in extrusive igneous rocks that is left by a gas bubble as the rock hardens.

Vitreous Luster – A shine resembling that of glass.

Volcano – A vent or fissure in the earth's crust through which molten rock and hot gasses escape.

Volcanic Bomb – Blob of lava thrown out of a volcano.  It solidifies before hitting the ground.

Vug – A cavity in rock, often lined with crystals.


Letters - W

Waxy – One of the technical terms used to refer to the luster of a gemstone.  Turquoise is an example of a gem with a waxy luster.    

Weathering – The breaking d down of rocks and minerals by the action of various natural processes.  Mechanical weathering, for example, occurs when heat, water, ice, and pressure act upon rock surfaces.  Chemical weather occurs when chemicals in the atmosphere or environment interact with the chemicals in rocks and minerals (carbon dioxide dissolved in rainwater produces a weak acid that can dissolve rock).


Letters - X

Xenolith – Country rock enclosed in magma.

Letters - Y

Yellowcake – The solid form of mixed uranium oxide, which is produced from uranium ore in the uranium recovery (milling) process.  The material is a mixture of uranium oxides, which can vary in proportion and color from yellow to orange to dark green (blackish) depending on the temperature at which the material is dried (which affects the level of hydration and impurities), with higher drying temperatures producing a darker and less soluble material.  The yellowcake produced by most modern mills is actually brown or black, rather than yellow, but the name comes from the color and texture of the concentrates produced by early milling operations.

Letters - Z

Zeolite – Zeolites are minerals that have a micro-porous structure.  Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater.  Zeolites also crystallized in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins.  Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz or other zeolites.


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