Gemstone Glossary

Gemstone Glossary

Whether you're a seasoned collector or just beginning your journey into the world of minerals and gemstones, understanding the terminology makes all the difference. Here's your go-to reference guide.

A

Agate
A banded variety of chalcedony (microcrystalline quartz) known for its colorful, layered patterns. Formed in volcanic rock cavities and prized for lapidary work, carvings, and cabochons.
Agatized
The process by which organic material (such as wood or bone) is replaced by agate over time, preserving the original structure in silica form.
Agatized Coral
Ancient coral that has been replaced by agate through the fossilization process, preserving the coral's intricate skeletal structure in colorful chalcedony. Found in Florida and other fossil-rich regions.
Ajax Turquoise
A turquoise mine located in Lander County, Nevada. Known for its rich blue color and considered a classic American turquoise source among collectors.
Allochromatic
A gemstone that is colorless in its pure form and gains color from trace impurities (e.g., emerald gets its green from chromium).
Aluminum
A lightweight, silver-white metal (element Al) used in jewelry and metalworking for its corrosion resistance and affordability. Aluminum is anodized in a wide range of colors and is popular in contemporary and craft jewelry. It is also a key component in minerals such as corundum, turquoise, and kyanite.
Amazonite
A blue-green variety of potassium feldspar (microcline) named after the Amazon River. Known for its striking teal to mint-green color, often with white streaking. Used in cabochons, beads, and carvings.
Amber
Fossilized tree resin, typically ranging from golden yellow to deep orange or brown. Not a mineral, but an organic gemstone. Prized for its warmth, translucency, and inclusions. Mohs hardness: 2–2.5.
Amethyst
A purple variety of quartz, ranging from pale lilac to deep violet. Color is caused by iron impurities and irradiation. Mohs hardness: 7.
Ammolite
An opalescent organic gemstone derived from the iridescent shell of ammonite fossils, primarily found in the Bearpaw Formation of Alberta, Canada. Displays vivid play of color in reds, greens, and golds. One of the rarest gemstones in the world.
Ammonite
An extinct group of marine cephalopod mollusks with a coiled shell, related to modern nautiluses. Among the most recognized and collected fossils worldwide.
Ammonite Fossil
The preserved shell of an ammonite, often found in limestone or shale. Some ammonites display iridescent shell material called ammolite, a gemstone in its own right.
Amorphous
A solid material that lacks a crystalline structure. Opal is a well-known amorphous gemstone, composed of hydrated silica spheres rather than a regular crystal lattice.
Andamooka Opal
A variety of opal from the Andamooka region of South Australia. Known for its crystal and matrix opals, often treated to enhance color and contrast.
Apatite
A group of phosphate minerals occurring in a wide range of colors. Blue apatite is particularly prized for its vivid neon to teal blue color. Mohs hardness: 5. Relatively soft for jewelry use, typically set in protective bezels.
Asterism
A star-like optical phenomenon seen in some gemstones caused by needle-like inclusions reflecting light.
Australian Opal
Opals sourced from Australia, which produces the majority of the world's precious opal supply. Includes black opals from Lightning Ridge, white/crystal opals from Coober Pedy, and boulder opals from Queensland.
Aventurine
A variety of quartz or feldspar containing reflective mineral inclusions that create a glittery shimmer called aventurescence. Most commonly green. Mohs hardness: 7.
Azurite
A vivid deep-blue copper carbonate mineral, often found alongside malachite. Prized as a mineral specimen and occasionally cut as cabochons. Mohs hardness: 3.5–4.

B

Banded Agate
A variety of agate displaying distinct, parallel bands or layers of alternating color or translucency. The banding is formed as silica-rich water slowly deposits concentric layers within a cavity. Among the most classic and widely collected agate forms.
Banding
The pattern of parallel layers or stripes seen in minerals such as agate, malachite, and onyx. Banding is caused by successive mineral deposition over time, with each layer reflecting subtle changes in chemistry or environment during formation.
Belemite (Belemnite)
An extinct group of cephalopods related to squid. Their bullet-shaped internal shells are commonly found as fossils in Mesozoic marine sediments.
Beryl
A mineral species including emerald (green), aquamarine (blue), morganite (pink), heliodor (yellow), and goshenite (colorless). Mohs hardness: 7.5–8.
Bezel
A jewelry setting style in which a band of metal completely surrounds and holds a gemstone. Offers excellent protection for the stone and a sleek, modern look.
Bisbee Turquoise
One of the most coveted American turquoises, mined from the Lavender Pit copper mine in Bisbee, Arizona. Known for its rich royal blue to blue-green color with distinctive chocolate-brown matrix. The mine is now closed, making genuine Bisbee turquoise increasingly rare.
Black Opal
The rarest and most valuable variety of opal, characterized by a dark body tone that makes play-of-color appear vivid and intense. Lightning Ridge, Australia is the most famous source.
Blastoid
An extinct class of echinoderms with a distinctive bud-shaped body. Found in Paleozoic marine sediments, prized by collectors for their intricate five-fold symmetry.
Blue Apatite
See Apatite. The vivid neon to teal-blue gem variety of the apatite mineral group. Popular in artisan jewelry for its electric color, typically set in bezel or prong settings due to its relative softness (Mohs 5).
Blue Gem Turquoise
A turquoise mine in Lovelock, Nevada, known for producing high-quality sky-blue to deep blue turquoise, often with little to no matrix.
Blue Opal
An opal with a blue body color. Notable sources include Peru and Oregon. Peruvian blue opal is prized for its soft, robin's-egg blue color.
Body Tone
In opals, the base color of the stone — ranging from white (light) to black (dark). Dark body tones generally show play-of-color more vividly.
Boulder Opal
A type of opal found in Queensland, Australia, where thin veins or patches of precious opal form within ironstone boulders. The ironstone matrix is left attached, serving as a natural backing. Boulder opals can display vivid play-of-color despite thin opal layers.
Boulder Turquoise
Turquoise left embedded in its host rock matrix. May appear as veins, patches, or nodules within surrounding stone, creating striking natural patterns valued by collectors and jewelers.
Brachiopod
A marine invertebrate with two shells, superficially resembling bivalve mollusks but unrelated. Extremely common in Paleozoic rocks and important index fossils for dating geological formations.
Brilliance
The amount of white light reflected from the interior and surface of a gemstone.
Bronze
An alloy primarily composed of copper and tin, one of the oldest metals used by humans. In jewelry making, bronze is valued for its warm golden-brown color, durability, and affordability. Develops a natural patina over time. Often used in artisan and bohemian-style jewelry.
Broken Arrow Turquoise
A turquoise mine in Nevada known for producing vivid blue to blue-green turquoise with attractive matrix patterns.

C

Cabochon
A gemstone shaped and polished with a smooth, rounded dome on top and a flat base — as opposed to faceted. Common for opals, turquoise, and apatite.
Calcite
A calcium carbonate mineral (CaCO₃) and one of the most common minerals on Earth. Makes up limestone, marble, and chalk. Mohs hardness: 3. Often fluorescent under UV light.
Cambrian Period
The first period of the Paleozoic Era, approximately 538–485 million years ago. Marked by the explosive diversification of multicellular animal life known as the Cambrian Explosion. Key fossil groups include trilobites, brachiopods, and early echinoderms.
Campitos Turquoise
Turquoise from the Cananea mining district in Sonora, Mexico. Known for its medium blue color, often with a tan or brown matrix.
Candelaria Turquoise
A Nevada turquoise mine known for producing light blue to blue-green stones, often with a white or tan matrix.
Carat (ct)
The unit of weight for gemstones. 1 carat = 0.2 grams. Not to be confused with karat, which measures gold purity.
Carboniferous Period
A period of the Paleozoic Era, approximately 359–299 million years ago. Named for the vast coal swamp forests that dominated the era. Fossils include ferns, early reptiles, amphibians, sharks, and abundant crinoids and brachiopods.
Carico Lake Turquoise
A Nevada turquoise mine famous for its rare lime green to apple green turquoise, caused by zinc in the mineral composition. Highly collectible.
Carnelian
A reddish-orange to deep red variety of chalcedony. One of the oldest used gemstones, prized in antiquity for seals and amulets. Mohs hardness: 7.
Castle Dome Turquoise
Turquoise from the Castle Dome area in Arizona. Known for its varied blue and green hues with interesting matrix patterns from the copper-rich host rock.
Cerrillos Turquoise
One of the oldest known turquoise mining sites in North America, located in the Cerrillos Hills of New Mexico. Mined for over 1,000 years by Native American peoples.
Chatoyancy
Also called the cat's eye effect — a band of reflected light across a gemstone surface caused by parallel fibrous inclusions.
Chisel
A hand tool with a sharpened metal blade used by rockhounds and fossil hunters to split rock along natural planes or fractures. Used with a crack hammer to expose mineral specimens and fossils embedded in host rock.
Chocolate Opal
An opal with a warm brown to chocolate-colored body tone, often from Lightning Ridge, Australia.
Chrysocolla
A blue-green copper silicate mineral often found alongside malachite, azurite, and turquoise. Mohs hardness: 2.5–3.5.
Chrysoprase
A green variety of chalcedony colored by nickel. Prized for its vivid apple-green to mint color. Mohs hardness: 7.
Citrine
A yellow to orange-brown variety of quartz, colored by iron. Natural citrine is relatively rare; much commercial citrine is heat-treated amethyst. Mohs hardness: 7.
Clarity
The degree to which a gemstone is free from internal inclusions or external blemishes.
Clear Turquoise
Turquoise that is free of matrix or host rock — a solid, uniform field of pure turquoise color with no veining or inclusions. Generally considered more valuable, especially in vivid blue specimens.
Cleavage
The tendency of a mineral to break along flat, parallel planes determined by its internal crystal structure.
Coober Pedy
A town in outback South Australia and the world's largest source of white/light opal. Known as the "Opal Capital of the World," Coober Pedy produces vast quantities of precious opal including crystal and white opal. Many miners live underground (in "dugouts") to escape the extreme heat.
Copper
A reddish-orange metallic element (Cu) that is both a native element mineral and a widely used jewelry metal. As a native element, copper occurs in nugget or wire form and is a prized mineral specimen. In jewelry, it is used on its own (developing a natural patina) and as the base for alloys like bronze and brass. Also a key coloring agent in minerals like malachite, azurite, turquoise, and chrysocolla.
Coral (Fossil)
The skeletal remains of ancient coral colonies preserved in rock. Fossil coral is commonly found in limestone formations and may be agatized, silicified, or calcified.
Corundum
A crystalline form of aluminum oxide (Al₂O₃) and one of the hardest minerals (Mohs 9). Gemstone varieties include ruby (red) and sapphire (all other colors).
Crack Hammer
A heavy, double-faced steel hammer used in rockhounding and fossil hunting to split rocks and break open nodules or concretions. Used in combination with a chisel to expose specimens. Also called a prospecting hammer or geological hammer.
Cretaceous Period
The final period of the Mesozoic Era, approximately 145–66 million years ago. Ended with the mass extinction event that wiped out non-avian dinosaurs. Fossils include dinosaurs, mosasaurs, plesiosaurs, ammonites, and flowering plants. Chalk and limestone formations from this era contain abundant marine fossils.
Crinoid
Marine invertebrates related to sea stars and sea urchins, sometimes called sea lilies. Their fossils are extremely common in Paleozoic limestone.
Crinoid Stems
The segmented, column-like stalks that attached crinoids to the seafloor. Disc-shaped stem segments are frequently collected and used in fossil jewelry.
Cripple Creek Turquoise
Turquoise associated with the famous Cripple Creek gold mining district in Teller County, Colorado. Rare and collectible.
Crystal
A solid material whose atoms are arranged in a highly ordered, repeating geometric pattern. Crystals form naturally through geological processes.
Crystal Opal
A variety of precious opal that is transparent to semi-transparent, allowing light to pass through the stone and illuminate the play-of-color from within. Crystal opal can have any body tone. The transparency gives the stone a glowing, luminous quality and is highly prized among opal collectors.

D

Damele Turquoise
A Nevada turquoise mine in Lander County known for producing unusual green, yellow-green, and variscite-like stones with a distinctive webbed or matrix-heavy appearance.
Density
The mass of a mineral per unit volume, typically expressed as specific gravity relative to water.
Devonian Period
A period of the Paleozoic Era, approximately 419–359 million years ago. Known as the "Age of Fishes" for the diversification of fish. Also saw the first forests, amphibians, and insects. Fossils include fish, corals, brachiopods, ammonites, and crinoids.
Diamond
The hardest natural mineral (Mohs 10), composed of pure carbon in a cubic crystal structure. Prized for its exceptional brilliance, fire, and durability. Found in kimberlite pipes and alluvial deposits worldwide.
Dioptase
A rare, vivid emerald-green copper cyclosilicate mineral. Highly prized as a mineral specimen for its intense color. Mohs hardness: 5.
Dop Stick
A wooden or metal dowel used in lapidary work to hold a gemstone preform securely while grinding and polishing on a lapidary wheel. The stone is attached to the dop stick using dopping wax. Essential tool for shaping cabochons accurately.
Druzy
A layer of tiny crystals that form on the surface of a mineral or geode cavity, creating a sparkling, sugar-like texture. Popular in jewelry for its natural glittery appearance.

E

Echinoid
A class of marine invertebrates (echinoderms) that includes sea urchins and sand dollars. Echinoid fossils are found worldwide in marine limestone formations, often beautifully preserved with five-fold symmetry.
Electroforming
A metalworking technique used in jewelry making where metal (usually copper or silver) is deposited onto a surface using an electrolytic process, creating intricate organic-looking settings.
Ethiopian Opal
Opal sourced from Ethiopia, primarily from the Welo and Shewa provinces. Known for its vivid play-of-color and hydrophane nature. Includes crystal, white, and black body tone varieties. Has become a major opal source since the early 2000s.

F

Facet
A flat, polished surface cut onto a gemstone to maximize light reflection and brilliance.
Feldspar
The most abundant mineral group in the Earth's crust. Includes gemstone varieties such as moonstone, sunstone, labradorite, and amazonite. Mohs hardness: 6–6.5.
Fire
The dispersion of light into spectral colors within a gemstone.
Fire Opal
A transparent to translucent opal with a vivid orange, red, or yellow body color, most famously sourced from Mexico. May or may not show play-of-color.
Float
In rockhounding, float refers to loose rocks or mineral fragments found on the surface that have weathered and eroded away from their original outcrop or vein. Finding float is often a clue that a productive mineral deposit is nearby uphill.
Fluorescence
The tendency of some gemstones to emit visible light when exposed to ultraviolet (UV) light.
Fluorite
A colorful calcium fluoride mineral. Famous for its perfect octahedral cleavage and strong fluorescence under UV light. Mohs hardness: 4.
Fossilization
The process by which the remains of an organism are preserved in rock over geological time, typically through mineral replacement, permineralization, or carbonization.
Fossilized Wood
Wood preserved through petrification or permineralization, with organic material replaced by silica or other minerals. Often retains the original grain and bark texture.
Fox Turquoise
A turquoise mine located in Lander County, Nevada. Known for its bright blue color and considered one of the classic collectible Nevada turquoise mines.
Fracture
The way a mineral breaks when it does not cleave along a flat plane. Types include conchoidal (shell-like), uneven, fibrous, and hackly (jagged).
Freeform
A gemstone or mineral shaped without adhering to a standard geometric form. Freeforms follow the natural contours of the rough material, resulting in unique pieces.

G

Garnet
A large group of silicate minerals varying widely in color — including red (almandine, pyrope), orange (spessartine), and green (tsavorite, demantoid). Mohs hardness: 6.5–7.5.
Gastropod
A class of mollusks that includes snails and slugs. Gastropod fossils — characterized by coiled, spiral shells — are found in a wide range of geological formations.
Gembone
Fossilized dinosaur or ancient animal bone replaced by minerals such as silica or opal over millions of years, creating gemstone-quality material prized by collectors and jewelers.
Geode
A hollow rock lined on the inside with crystals or mineral matter. Common crystal types include quartz, amethyst, and calcite.
Glass (Natural / Obsidian)
Natural glass is an amorphous solid formed when silica-rich lava cools rapidly. Obsidian is the most common natural glass.
Gold
A precious metal (element Au) prized since antiquity for its rarity, malleability, and resistance to tarnish. Occurs as a native element in quartz veins and alluvial deposits. Used extensively in fine jewelry. Purity is measured in karats (24k = pure gold).
Golden Hills Turquoise
A turquoise mine from Kazakhstan known for producing distinctive light blue to sky-blue turquoise with a cream or tan matrix.
Graptolite
An extinct colonial marine organism that lived from the Cambrian through the Carboniferous period. Graptolite fossils appear as delicate, saw-blade-like markings on dark shale. They are important index fossils used to date Ordovician and Silurian rock formations.
Green Opal
An opal with a green body color, ranging from pale mint to vivid emerald tones. Sources include Tanzania, Peru, and Australia.
Gypsum
A soft calcium sulfate mineral (Mohs hardness: 2). Varieties include selenite (clear/crystalline), alabaster (massive/white), and satin spar (fibrous).

H

Hachita Turquoise
Turquoise from the Hachita mining district in Grant County, New Mexico. Known for its blue to blue-green color with varied matrix.
Hardness (Mohs Scale)
A scale from 1–10 measuring a mineral's resistance to scratching. Diamond is 10; talc is 1.
Heat Treated
A gemstone enhancement process in which a stone is exposed to controlled high temperatures to alter, improve, or stabilize its color. Common in amethyst (turned to citrine), sapphire, ruby, and tanzanite. Heat treatment is widely accepted in the gem trade and should be disclosed.
Horn Coral
A type of extinct solitary coral (Rugosa) named for its horn or cone shape. Common in Paleozoic marine limestone and popular collector specimens.
Howlite
A white to light grey calcium borosilicate mineral with distinctive grey or black veining. Often dyed blue or turquoise and sold as a simulant for turquoise. In its natural undyed form, howlite is collected for its own subtle beauty. Mohs hardness: 3–3.5.
Hubei Turquoise
Turquoise from Hubei Province in central China, one of the world's most productive turquoise mining regions. Ranges from sky blue to green, often with black spider web matrix.
Hydrophane
A type of opal that is porous and can absorb water, temporarily changing its appearance when wet. Common in Ethiopian Welo opals.

I

Ichthyosaur
An extinct group of large marine reptiles that dominated the Mesozoic seas. Their fossils — resembling a cross between a dolphin and a fish — are found in Triassic through Cretaceous formations. Ichthyosaur remains are among the most dramatic and collectible Mesozoic fossils.
Igneous
Rock formed through the cooling and solidification of magma or lava. Can be intrusive (e.g., granite) or extrusive (e.g., basalt).
Inclusion
Any internal characteristic within a gemstone — such as crystals, fractures, or gas bubbles — formed during the stone's growth.
Inlay
A decorative technique in jewelry and lapidary work in which pieces of stone, shell, or other material are set flush into a recessed surface of metal or another stone to create a flat, mosaic-like design. Common in Southwestern Native American jewelry using turquoise, coral, shell, and jet.
Iridescence
A surface optical effect that causes color to shift as the viewing angle changes.

J

Jade
A term covering two distinct minerals: nephrite and jadeite. Both are prized for their toughness and beauty, historically revered in Chinese, Mesoamerican, and Maori cultures.
Jasper
An opaque variety of chalcedony that occurs in virtually every color and pattern — red, yellow, brown, green, and multicolored. Mohs hardness: 7.
Jet
A black organic gemstone formed from ancient driftwood compressed under extreme pressure over millions of years — essentially a form of coal. Prized in Victorian mourning jewelry for its deep black color and ability to take a high polish. Lightweight and warm to the touch. Mohs hardness: 2.5–4.
Jurassic Period
The middle period of the Mesozoic Era, approximately 201–145 million years ago. Famous for the dominance of large dinosaurs and the appearance of the first birds. Marine fossils include ammonites, belemnites, ichthyosaurs, and plesiosaurs. Morrison Formation in the American West is a key Jurassic fossil site.

K

Kingman Turquoise
One of the most prolific and well-known American turquoise mines, located in Mohave County, Arizona. Produces turquoise in a wide range of blues and greens, often with a black or silver spider web matrix. Still in active production.
Kings Manassa Turquoise
Turquoise from the Manassa (King) mine in Conejos County, Colorado. Historically worked by Native Americans. Produces distinctive green to blue-green turquoise with a golden or brown matrix.
Kyanite
A blue aluminum silicate mineral known for its unusual variable hardness — Mohs 4–5 along the crystal length, and 6–7 across it. Prized for its vivid blue color in mineral specimens and jewelry.

L

Labradorite
A feldspar mineral prized for its striking optical phenomenon called labradorescence — a vivid play of iridescent color (blue, green, gold, orange) caused by light interference within the stone's layered structure. Mohs hardness: 6–6.5.
Lander Blue Turquoise
Often considered the rarest and most valuable American turquoise, from a small deposit in Lander County, Nevada. Characterized by its intense medium-blue color with a distinctive black spider web matrix. The original deposit is now essentially mined out.
Lapidary
The art and craft of cutting, shaping, and polishing gemstones and minerals. A lapidary (person) works with rough material to produce cabochons, faceted gems, carvings, and slabs.
Lapidary Saw
A specialized power saw used to cut rough mineral specimens and gemstones. Typically uses a diamond-impregnated circular blade and a cooling lubricant (water or oil). Types include slab saws (for large slices), trim saws (for smaller cuts), and wire saws (for delicate work).
Lapis Lazuli
A deep-blue metamorphic rock prized as a gemstone and pigment for thousands of years. Composed primarily of lazurite, often with golden pyrite inclusions and white calcite veining. Mined chiefly in Afghanistan, Chile, and Russia. Mohs hardness: 5–6.
Larimar
A rare blue variety of pectolite found only in the Dominican Republic. Known for its ocean-blue to turquoise color with white patterns resembling waves. Mohs hardness: 4.5–5.
Leadville Turquoise
Turquoise associated with the Leadville mining district in Lake County, Colorado. A historic and collectible American turquoise source.
Lepidolite
A lilac to pink lithium mica mineral, often found with tourmaline and other lithium-rich minerals. Mohs hardness: 2.5–3.
Lightning Ridge Opal
Precious opal from Lightning Ridge, New South Wales, Australia. The world's premier source of black opal, known for its dark body tone and extraordinarily vivid play-of-color.
Lone Mountain Turquoise
A Nevada turquoise mine considered one of the finest American turquoise sources. Produces rich, deep blue turquoise with a distinctive black spider web matrix. Now closed, making genuine specimens rare and highly sought after.
Loupe
A small magnification lens used by gemologists, rockhounds, and jewelers to examine stones closely. Standard gemological loupes provide 10x magnification and are used to evaluate inclusions, surface features, and craftsmanship. Essential tool for field identification and gem grading.
Luster
The way light interacts with a gemstone's surface — described as vitreous, resinous, silky, pearly, or adamantine.

M

Malachite
A vivid green copper carbonate mineral with distinctive banded patterns. Prized both as a mineral specimen and for lapidary use. Often associated with azurite and chrysocolla. Mohs hardness: 3.5–4.
Matrix
The host rock in which a gemstone or mineral is embedded. In opal, matrix refers to opal that occurs within and throughout its host rock (commonly ironstone or sandstone).
Matrix Opal
Opal that permeates and is intergrown with its host rock rather than forming a distinct separate layer or nodule. The opal and host rock cannot be separated. Common in Queensland boulder opal and Andamooka matrix opal (which is often treated to darken the background and enhance color).
Mesozoic Era
A geologic era spanning approximately 252–66 million years ago, divided into the Triassic, Jurassic, and Cretaceous periods. Known as the "Age of Reptiles," this era saw the rise and extinction of dinosaurs, the evolution of birds and early mammals, and the proliferation of ammonites and marine reptiles.
Metamorphic
Rock that has been transformed from its original form by heat, pressure, or chemically active fluids deep within the earth. Examples include marble and schist.
Meteorite
A solid piece of debris from space (an asteroid, comet, or other body) that survives its passage through Earth's atmosphere and lands on the surface. Meteorites are classified as stony, iron, or stony-iron. Iron meteorites, with their distinctive Widmanstätten pattern, and pallasite meteorites (containing olivine crystals in an iron-nickel matrix) are highly prized as collector specimens and used in jewelry.
Mineral
A naturally occurring, inorganic solid with a definite chemical composition and an ordered crystalline structure. Minerals are the building blocks of rocks.
Mineral Specimen
A naturally occurring piece of mineral or crystal, valued for its natural form rather than being cut or polished.
Mississippian Period
The early part of the Carboniferous Period (approximately 359–323 million years ago) as defined in North America. Known as the "Age of Crinoids" for the extraordinary abundance of crinoid fossils in Mississippian limestone formations across the American Midwest.
Moldavite
A rare and highly collectible natural glass formed approximately 15 million years ago from the impact of a meteorite in what is now Germany. The heat and pressure of impact melted both the meteorite and surrounding rock, scattering glassy fragments across Central Europe — primarily in the Czech Republic's Bohemian region. Moldavite is olive to forest green, translucent, with a characteristic etched, bubbly surface texture. Prized by collectors and used in jewelry. Mohs hardness: 5–5.5.
Mollusk
A large phylum of invertebrate animals including snails (gastropods), clams (bivalves), squid, octopus, and ammonites (cephalopods). Mollusk shells are among the most abundant fossils found in sedimentary rock worldwide.
Moonstone
A feldspar mineral prized for its adularescence — a floating, billowy glow of blue or white light caused by light scattering between microscopic feldspar layers. Mohs hardness: 6–6.5.
Mookaite (Mookite)
An Australian variety of radiolarite (silicified radiolarian sedimentary rock) found near Mooka Creek in Western Australia. Known for its striking combination of reds, yellows, purples, and creams. Popular in lapidary work and prized by collectors for its bold, earthy color palette.
Morenci Turquoise
Turquoise from the Morenci copper mine in Greenlee County, Arizona. Known for its vivid blue color and distinctive blue-black or silver pyrite matrix, creating a striking spider web effect.

N

Native Element
A mineral composed of a single element in its natural state. Examples include gold, silver, copper, sulfur, and diamond (carbon).
Natural vs. Synthetic
Natural gemstones are formed by geological processes in the earth. Synthetic (lab-created) gemstones have the same chemical composition and crystal structure as their natural counterparts but are grown in a laboratory. Simulants are different materials that merely resemble a gemstone in appearance. Disclosure of treatment, synthesis, or simulation is expected in the gem trade.
New Lander Turquoise
A turquoise deposit in Lander County, Nevada, related to but distinct from the original Lander Blue mine. Shares some characteristics of Lander Blue — rich blue color and spider web matrix.
Nodule
A rounded, compact mass of mineral matter that forms within sedimentary rock, often with a different composition than the surrounding rock. Nodules frequently contain fossils, crystals, or minerals within — including flint, chert, pyrite, septarian calcite, and occasionally opal. Rockhounds often crack open nodules to discover what's inside.
Number 8 Turquoise
A legendary Nevada turquoise mine named for a mining claim number. Famous for its golden-brown to black spider web matrix over a rich blue or blue-green body — one of the most distinctive turquoise patterns in the world. The original mine is now closed.

O

Obsidian
A naturally occurring volcanic glass formed when lava cools rapidly. Typically black, but can be brown, gray, or green. Has a conchoidal fracture. Mohs hardness: 5–5.5.
Onyx
A banded variety of chalcedony (microcrystalline quartz) typically displaying parallel bands of black and white. True black onyx (a solid black chalcedony) is often dyed to achieve uniform color. Widely used in jewelry, carvings, and inlay work. Mohs hardness: 7.
Opalized
Describes organic material (such as wood, shell, or bone) that has been replaced by opal during fossilization, preserving the original structure in precious or common opal form.
Ordovician Period
A period of the Paleozoic Era, approximately 485–444 million years ago. Ended with one of Earth's five major mass extinctions. Fossils include trilobites, graptolites, brachiopods, crinoids, and early corals.
Outcrop
A visible exposure of bedrock or ancient rock formation at the Earth's surface. Outcrops are prime targets for rockhounds and geologists because they reveal the underlying geology and may expose mineral veins, fossil beds, or gemstone-bearing rock without digging.

P

Paleozoic Era
A geologic era spanning approximately 538–252 million years ago, divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvanian in North America), and Permian. Characterized by the evolution of complex animal life in the oceans and later the colonization of land. Ended with the largest mass extinction in Earth's history.
Patina
A surface layer or finish that develops on metal over time through oxidation or chemical treatment. In jewelry, patina is often intentionally applied to silver or copper to create an antique, aged appearance.
Pennsylvanian Period
The later part of the Carboniferous Period (approximately 323–299 million years ago) as defined in North America. Characterized by vast coal-forming swamp forests. Fossils include giant insects, early reptiles, ferns, and abundant marine invertebrates.
Peridot
The gem variety of the mineral olivine, occurring in a distinctive olive to lime green color. Mohs hardness: 6.5–7. Notable sources include Egypt, Pakistan, and Arizona.
Permian Period
The final period of the Paleozoic Era, approximately 299–252 million years ago. Ended with the largest mass extinction in Earth's history, wiping out over 90% of marine species. Fossils include early reptiles (ancestors of dinosaurs and mammals), brachiopods, and ammonoids.
Permineralization
A common fossilization process in which mineral-rich groundwater permeates the pores of bone, wood, or shell and deposits minerals — most commonly silica, calcite, or pyrite — filling the spaces while preserving the original structure. Petrified wood and dinosaur bone are classic examples.
Persian / Nishapur Turquoise
Turquoise from the Nishapur region of Iran (historically called Persia), considered by many to be the finest turquoise in the world. Prized for its pure, robin's-egg blue color, typically with little to no matrix.
Petoskey Stone
A fossilized colonial coral (Hexagonaria percarinata) found primarily around Lake Michigan in Michigan, the official state stone. When polished, the distinctive hexagonal coral polyp pattern becomes beautifully visible.
Petrification
A type of fossilization in which organic material is slowly replaced by minerals (commonly silica or calcite), turning biological structures to stone.
Petrified Wood
Ancient wood that has been turned to stone through permineralization, where silica gradually replaces the organic wood cells, often preserving the original wood grain and bark in stunning detail.
Peruvian Blue Opal
A common (non-precious) opal found in Peru, prized for its soft, robin's-egg to turquoise blue color. Unlike precious opal, it does not display play-of-color but is valued for its gentle, even hue and translucency. Popular in cabochons and beads.
Pilot Mountain Turquoise
A turquoise mine in Mineral County, Nevada, known for producing high-quality blue to blue-green turquoise with dark matrix.
Play-of-Color
The optical phenomenon unique to precious opals — flashes of spectral color caused by the diffraction of light through silica spheres.
Pleochroism
The property of some gemstones to display different colors when viewed from different angles.
Preform
A rough gemstone or mineral that has been shaped into an approximate final form (usually dome-shaped for a cabochon) before final finishing and polishing. Creating a preform on a grinding wheel is a key step in lapidary work, allowing the lapidary to establish the outline and dome height before moving to finer grits.
Prong Setting
A jewelry setting style that uses metal claws (prongs) to grip and hold a gemstone in place. Maximizes light exposure to the stone.
Pyrite
An iron sulfide mineral (FeS₂) nicknamed "fool's gold" for its metallic, golden appearance. Pyrite forms cubic crystals and is a popular mineral specimen.

Q

Quartz
One of the most abundant minerals on Earth, composed of silicon dioxide (SiO₂). Major gem varieties include amethyst, citrine, rose quartz, smoky quartz, and clear (rock) crystal. Mohs hardness: 7.
Quartzite
A metamorphic rock formed when quartz sandstone is subjected to intense heat and pressure. Extremely hard and durable, used in architecture, lapidary, and decorative arts.

R

Refractive Index (RI)
A measure of how much light bends as it passes through a gemstone. Each mineral has a characteristic RI that gemologists use as a key identification property. Measured with a refractometer, RI helps distinguish between stones of similar appearance (e.g., natural sapphire vs. synthetic, or glass simulants).
Rhodochrosite
A manganese carbonate mineral prized for its vivid pink to rose-red color and distinctive banded patterns. Mohs hardness: 3.5–4.
Rhodonite
A pink manganese silicate mineral, often with distinctive black manganese oxide veining. Used as a decorative stone and in cabochons. Mohs hardness: 5.5–6.5.
Rock
A naturally occurring solid aggregate of one or more minerals. Rocks are classified into three main types: igneous, sedimentary, and metamorphic.
Rockhounding
The hobby of searching for, collecting, and studying rocks, minerals, gemstones, and fossils in their natural settings. Rockhounds use maps, geological surveys, and field tools to locate productive sites on public lands. A rewarding outdoor pursuit that combines geology, history, and treasure hunting.
Rose Quartz
A pink variety of quartz, ranging from pale blush to deep rose. Mohs hardness: 7.
Rough / Raw
A gemstone or mineral in its natural, uncut, and unpolished state as extracted from the earth.
Royston Turquoise
Turquoise from the Royston mining district in Nye County, Nevada. Known for its wide range of colors — from rich green to teal to deep blue — often with a golden-brown to tan matrix.

S

925 Sterling Silver
An alloy composed of 92.5% pure silver and 7.5% other metals (typically copper). The standard for quality silver jewelry worldwide. The "925" stamp is a hallmark guaranteeing this composition.
Santa Rita Turquoise
Turquoise from the Santa Rita copper mining area in Grant County, New Mexico. A historic and collectible turquoise source in the American Southwest.
Sediment
Particles of rock, mineral, or organic matter transported and deposited by water, wind, or ice. Sediment accumulates in layers over time and can eventually form sedimentary rock.
Sedimentary
Rock formed from the compaction and cementation of sediment layers over time. Examples include sandstone, limestone, and shale.
Selenite
A transparent to translucent crystalline variety of gypsum (calcium sulfate). Forms large, blade-like crystals and is prized for its silky, pearlescent luster. Mohs hardness: 2.
Septarian
A type of concretion or nodule containing angular cracks filled with minerals — most commonly yellow calcite, brown aragonite, and grey limestone. The distinctive geometric patterning resembles a turtle shell or stained glass. Found in marine sedimentary rock, septarian nodules are popular display specimens and are used in lapidary work and home décor.
Shark Tooth (Fossil)
One of the most commonly found and collected fossils worldwide. Shark teeth fossilize readily because they are composed of dense, mineralized tissue. Found in marine sedimentary deposits from the Devonian to the Pleistocene. Megalodon teeth are among the most prized collector specimens.
Sierra Nevada Turquoise
Turquoise associated with the Sierra Nevada mining region. Known among collectors for its distinctive color characteristics reflecting the geology of the area.
Sieve
A mesh screen or graded series of screens used by rockhounds to sort material by size when searching for small minerals, gemstones, or fossil fragments in gravel, sand, or tailings. Sieving (or screening) is a common technique at gem mines, fossil beds, and stream placers.
Silicate
The largest and most common group of minerals, all containing silicon and oxygen (SiO₄) as their basic building block. Silicate minerals make up over 90% of the Earth's crust.
Silurian Period
A period of the Paleozoic Era, approximately 444–419 million years ago. Saw the colonization of land by vascular plants and the diversification of jawed fish. Marine fossils include corals, crinoids, brachiopods, trilobites, and graptolites.
Silver
A lustrous white precious metal (element Ag) used extensively in jewelry making. As a native element, silver occurs naturally in nugget or wire form and is a prized mineral specimen.
Sinai Turquoise
Turquoise from the Sinai Peninsula of Egypt, one of the oldest known turquoise mining sites in the world. Mined by ancient Egyptians for thousands of years.
Slab
A flat slice cut from a mineral specimen or rough gemstone, often used for display or further lapidary work.
Sleeping Beauty Turquoise
Turquoise from the Sleeping Beauty Mine in Globe, Arizona. Famous for its pure, robin's-egg blue color with little to no matrix. The mine closed in 2012, making genuine specimens increasingly scarce and valuable.
Smoke Treated
A treatment process used primarily on opals (especially Ethiopian opals) in which the stone is wrapped in paper and burned/smoked, causing carbon particles to penetrate the porous structure and darken the body tone. Smoke treatment can dramatically improve an opal's play-of-color by creating a dark background, simulating black opal. The treatment is considered stable but should be disclosed.
Smoky Quartz
A brown to black variety of quartz, colored by natural irradiation of aluminum impurities. Mohs hardness: 7.
Sodalite
A blue tectosilicate mineral often confused with lapis lazuli. Characterized by its royal blue color with white calcite veining. Mohs hardness: 5.5–6.
Specific Gravity (SG)
The ratio of a gemstone's density to that of water — a key property used in gem identification.
Spider Web Turquoise
Turquoise with a fine, intricate web-like pattern of matrix running throughout the stone, resembling a spider's web. Spider web patterning — especially in black matrix — is highly prized. Famous spider web mines include Lander Blue, Number 8, and Lone Mountain.
Spinel
A magnesium aluminum oxide mineral occurring in a wide range of colors. Historically confused with ruby and sapphire. Mohs hardness: 8.
Stabilized
A turquoise (or other porous stone) treatment in which epoxy resin, acrylic, or other material is injected under pressure into the stone's pores to harden it and improve color consistency. Stabilization is one of the most common treatments for lower-grade turquoise and should always be disclosed. Stabilized turquoise is more affordable and durable than untreated natural turquoise.
Streak
The color of a mineral's powder, observed by rubbing the mineral against an unglazed porcelain streak plate. A more reliable identifier than surface color.
Sunstone
A variety of feldspar exhibiting aventurescence, caused by reflective platelets of hematite or copper within the stone. Mohs hardness: 6–6.5.

T

Tailings
The waste rock, gravel, and debris left over from mining operations after ore or gem material has been extracted. Tailings piles are popular rockhounding sites because they often contain overlooked mineral specimens, gemstones, and fossils discarded during the original mining process.
Tanzanite
A blue-violet variety of the mineral zoisite, found only near Mount Kilimanjaro in Tanzania. Strongly trichroic. Mohs hardness: 6–7. Discovered in 1967.
Tektite
A natural glass formed by the heat and pressure of a meteorite impact, which melts terrestrial rock and scatters glassy debris across large areas called strewn fields. Unlike meteorites, tektites are composed of Earth material transformed by the impact event. Moldavite is the most prized tektite; others include australites, philippinites, and indochinites.
Tiger's Eye
A chatoyant variety of quartz with golden to brown color. Named for its resemblance to a cat's eye, displaying a shifting band of light. Mohs hardness: 7.
Titanium
A strong, lightweight, silver-grey metal (element Ti) increasingly used in contemporary jewelry for its durability, hypoallergenic properties, and ability to be anodized in a wide spectrum of vivid colors (blue, purple, green, gold) through controlled oxidation. Harder than steel but lighter, making it ideal for large statement pieces.
Topaz
A silicate mineral occurring in a wide range of colors including colorless, blue, yellow, orange, pink, and the rare imperial topaz (orange-red). Mohs hardness: 8.
Tourmaline
A complex boron silicate mineral group with the widest color range of any gemstone — including watermelon (pink and green), indicolite (blue), rubellite (red/pink), chrome (vivid green), and paraiba (neon blue-green). Mohs hardness: 7–7.5.
Triassic Period
The first period of the Mesozoic Era, approximately 252–201 million years ago. Followed the largest mass extinction in Earth's history. Saw the rise of the first dinosaurs, crocodilians, and early mammals. Fossils include early reptiles, ammonites, and marine ichthyosaurs.
Trilobite
An extinct class of marine arthropods that lived from the Cambrian to the Permian period. Among the most iconic and widely collected fossils, prized for their intricate segmented bodies and compound eyes.
Turquoise
A blue-to-green opaque mineral (copper aluminum phosphate) prized for thousands of years in jewelry and ornamentation. Notable sources include the American Southwest, Iran, and Egypt. Mohs hardness: 5–6.
Turquoise Mountain Turquoise
Turquoise from the Turquoise Mountain mine in Mohave County, Arizona, one of the oldest and most historically significant turquoise mining sites in the American Southwest.
Tyrone Turquoise
Turquoise from the Tyrone copper mine in Grant County, New Mexico. Known for producing blue to blue-green stones with varied matrix patterns.

U

Unakite
A mottled green and pink rock composed of epidote, orthoclase feldspar, and quartz. Named after the Unaka Mountains in Tennessee. Popular in lapidary work for its earthy color combination. Mohs hardness: 6–7.

V

Vanadinite
A vivid red to orange-red lead vanadate chloride mineral forming small, hexagonal prismatic crystals on a matrix of barite or limonite. A highly prized mineral specimen, notably from Morocco and the American Southwest. Its striking color and geometric crystals make it a favorite among collectors. Not suitable for jewelry due to its lead content and low hardness (Mohs 3).
Variscite
A green phosphate mineral often confused with turquoise due to its similar color and occurrence. Ranges from pale mint to vivid apple green, typically without matrix veining. Variscite is softer than turquoise (Mohs 3.5–4.5) and is valued by collectors and used in cabochons and carvings. Notable sources include Utah and Nevada.
Villa Grove Turquoise
Turquoise from the Villa Grove area in Saguache County, Colorado. A historic Colorado turquoise source, known among collectors for its distinctive blue-green stones.

W

Water Opal
A transparent to near-transparent precious opal with a colorless or very pale body tone. The clarity of the stone allows the play-of-color to appear to float within the gem. Also called jelly opal. Found in Mexico and Ethiopia. Prized for its ethereal, glowing appearance.
Welo Opal
A variety of Ethiopian opal sourced from the Welo Province, known for its vivid play-of-color, hydrophane nature, and wide range of body tones.
White Buffalo Turquoise
A white to cream-colored stone with black matrix, mined in Nevada. Technically not traditional turquoise (composed of calcite and aluminum rather than copper), but mined and sold alongside turquoise and used similarly in jewelry. Prized in Native American and Western jewelry traditions.
Wire Wrapping
A jewelry-making technique in which wire (commonly sterling silver, gold-filled, or copper) is coiled, twisted, and woven around a gemstone or bead to create a setting without the use of heat or solder.