Rock IdentifierRock & mineral glossary
Key terms explained simply, A to Z.
A
- Accessory mineral
- A minor mineral present in small amounts in a rock, not essential to its classification, such as zircon or apatite in granite.
- Acicular
- A crystal habit in which crystals grow as fine, slender needles.
- Adamantine
- The brilliant, diamond-like luster of the most highly reflective minerals, such as diamond and cerussite.
- Agate
- A banded variety of chalcedony (microcrystalline quartz), prized for its colorful concentric layers.
- Aggregate
- A mass of many crystals or mineral grains grown together; most rocks are mineral aggregates.
- Alabaster
- A fine-grained, translucent form of gypsum (or sometimes calcite) soft enough to carve, long used for sculpture.
- Amorphous
- Lacking any orderly internal atomic structure; amorphous solids like opal and glass are mineraloids, not true minerals.
- Amygdaloidal
- A texture of volcanic rock whose former gas bubbles have been filled with later minerals such as quartz or calcite.
- Anhedral
- Describes crystals with no well-formed faces because they grew in cramped space, the opposite of euhedral.
- Aphanitic
- A fine-grained igneous texture in which crystals are too small to see without magnification, as in basalt.
- Asterism
- A star-shaped glow of light across a polished gem, caused by reflections from tiny aligned inclusions, as in star sapphire.
- Aventurescence
- A glittery sparkle caused by light reflecting from tiny plate-like inclusions, seen in aventurine and sunstone.
B
- Banded
- Showing parallel or concentric layers of differing color or composition, as in agate and gneiss.
- Bedding
- The layering in sedimentary rock formed as sediment settled in successive sheets.
- Birefringence
- The splitting of light into two rays as it passes through many crystals; strong birefringence makes calcite show a double image.
- Botryoidal
- A rounded, grape-like or bubbly surface habit formed by radiating crystals, common in malachite and hematite.
C
- Cabochon
- A gemstone cut with a smooth, domed top and flat base rather than facets, often used for opaque or star stones.
- Carat
- A unit of gemstone weight equal to 0.2 grams. Not to be confused with karat, a measure of gold purity.
- Chalcedony
- A form of quartz made of microscopic crystals; includes agate, carnelian, and jasper.
- Chatoyancy
- A moving band of light like a cat's eye, caused by reflections from parallel fibers or inclusions.
- Cleavage
- The tendency of a mineral to break along flat planes of atomic weakness, described by direction and angle.
- Color-change
- A gem's apparent shift in color under different lighting, famously in alexandrite (green in daylight, red under incandescent light).
- Conchoidal fracture
- A smooth, curved, shell-like breakage surface seen in quartz and obsidian, which lack cleavage.
- Concretion
- A hard, often rounded mass that forms when minerals cement sediment around a nucleus, sometimes mistaken for a fossil egg.
- Cryptocrystalline
- Made of crystals so small they can only be resolved under a microscope, as in chalcedony.
- Crystal
- A solid whose atoms are arranged in an orderly, repeating pattern, often expressed as flat faces and geometric form.
- Crystal system
- One of seven categories (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic) grouping crystals by their internal symmetry.
D
- Dendritic
- A branching, tree- or fern-like pattern, often formed by manganese oxides growing along cracks in a rock.
- Diaphaneity
- The degree to which a mineral transmits light: transparent, translucent, or opaque.
- Dispersion
- The splitting of white light into spectral colors by a gem, producing the flashes of 'fire' seen in diamond.
- Doublet
- An assembled stone made of two layers glued together, such as a slice of opal bonded to a dark backing.
- Druse
- A surface or cavity coated with many tiny projecting crystals; the adjective is 'druzy'.
- Druzy
- A sparkling crust of many tiny crystals coating a surface or cavity.
E
- Effervescence
- The fizzing of carbonate minerals like calcite when they react with weak acid, releasing carbon dioxide.
- Erosion
- The transport of weathered rock and sediment by water, wind, or ice — distinct from weathering, which breaks rock down in place.
- Euhedral
- Describes a crystal with well-developed, sharp faces because it grew with room to form freely.
F
- Feldspar
- The most abundant mineral group in Earth's crust; a key ingredient of granite and many other rocks.
- Felsic
- Describes light-colored igneous rocks rich in silica and minerals like quartz and feldspar, such as granite.
- Fire
- A gemologist's term for dispersion — the rainbow flashes a well-cut transparent gem throws from white light.
- Fluorescence
- The glow a mineral emits under ultraviolet light, caused by absorbing UV and re-emitting visible color.
- Foliation
- The banded or layered texture of metamorphic rock produced when minerals align under directed pressure.
- Fool's gold
- A nickname for pyrite (and sometimes chalcopyrite), whose brassy shine is mistaken for gold.
- Fracture
- The way a mineral breaks when it does not cleave, described as conchoidal, uneven, splintery, or hackly.
- Fusion crust
- The thin, dark, glassy rind formed on a meteorite as its surface melts during atmospheric entry.
G
- Gangue
- The commercially worthless minerals surrounding valuable ore in a deposit.
- Gem
- A mineral or organic material beautiful and durable enough to be cut and worn; a value label, not a scientific class.
- Geode
- A hollow rock lined inside with inward-pointing crystals or banded agate.
H
- Habit
- The characteristic external shape a crystal or mineral tends to grow into, such as prismatic, tabular, or massive.
- Hackly
- A jagged fracture with sharp, torn edges, like broken metal — typical of native copper.
- Heft
- The informal sense of how heavy a specimen feels for its size — a rough gauge of specific gravity.
- Hydrothermal
- Formed by hot, mineral-rich water; hydrothermal veins deposit many quartz, ore, and gem crystals.
I
- Igneous
- Rock formed by the cooling and solidification of molten magma or lava.
- Imitation
- A material that mimics a gem's look without its composition, such as glass or plastic standing in for a real stone.
- Inclusion
- A foreign material — crystal, liquid, or gas bubble — trapped inside a mineral; inclusions can help prove a stone is natural.
- Iridescence
- Rainbow color play caused by light interference in thin layers or microstructures, seen in labradorite and some obsidian.
J
- Jasper
- An opaque, solid-colored variety of chalcedony (microcrystalline quartz), often red, brown, or green.
L
- Labradorescence
- The vivid metallic blue-green flash of labradorite, caused by light scattering within its internal layering.
- Lapidary
- The craft of cutting, grinding, and polishing stone, and a person who practices it.
- Lithification
- The process turning loose sediment into solid rock through compaction and cementation.
- Luminescence
- The general emission of light by a mineral without strong heat, including fluorescence and phosphorescence.
- Luster
- The way a mineral's surface reflects light, described as metallic, vitreous, adamantine, pearly, silky, resinous, or dull.
M
- Mafic
- Describes dark igneous rocks rich in magnesium and iron, such as basalt and gabbro.
- Magma
- Molten rock beneath Earth's surface; when it erupts it is called lava.
- Massive
- A habit describing a mineral with no visible crystal shape — a solid, formless lump.
- Matrix
- The host rock in which crystals or gems are embedded; attractive matrix can raise a specimen's value.
- Metamorphic
- Rock formed when existing rock is transformed in the solid state by heat and pressure.
- Mineral
- A naturally occurring, inorganic solid with a definite chemical composition and orderly atomic structure.
- Mineraloid
- A naturally occurring mineral-like solid that lacks a crystal structure, such as opal, obsidian, and amber.
- Mohs hardness
- A 1–10 scale of a mineral's resistance to scratching, from talc (1) to diamond (10).
N
- Native element
- A mineral made of a single chemical element in uncombined form, such as gold, copper, sulfur, or diamond.
- Nephrite
- One of the two minerals called jade; a tough amphibole, more common than jadeite.
- Nodule
- A rounded lump of mineral matter that is solid throughout, unlike a hollow geode.
O
- Opalescence
- A soft, milky internal glow like that of common opal or moonstone, distinct from opal's flashing play-of-color.
- Ore
- A rock or mineral deposit containing enough valuable metal to be worth mining.
P
- Parting
- Breakage along planes of weakness that resemble cleavage but arise from twinning or strain, not the basic atomic structure.
- Permineralization
- A form of fossilization in which minerals fill and replace the pores of buried remains, turning them to stone.
- Phaneritic
- A coarse-grained igneous texture with crystals large enough to see, indicating slow, deep cooling, as in granite.
- Phenocryst
- A large crystal set within a finer-grained rock, giving a porphyritic texture and recording two cooling stages.
- Phosphorescence
- A lingering afterglow some minerals emit for seconds after a UV light is turned off.
- Placer
- A concentration of heavy, durable minerals (gold, gemstones) sorted and deposited by flowing water in stream gravels.
- Play-of-color
- The shifting spectral flashes of precious opal, produced as light diffracts through its ordered silica spheres.
- Pleochroism
- The property of showing different colors when a crystal is viewed from different directions, as in tanzanite and iolite.
- Porphyritic
- An igneous texture with large crystals set in a fine-grained background, recording two stages of cooling.
- Potch
- Common opal that lacks play-of-color, showing only a solid body color.
- Pseudomorph
- A crystal that keeps the outward shape of one mineral while its interior has been replaced by another.
Q
- Quartz
- Silicon dioxide (SiO₂), one of the most common minerals; varieties include amethyst, citrine, and rose quartz.
R
- Refractive index
- A measure of how much a gem bends light passing through it; a key, precise property used by gem labs to identify stones.
- Regmaglypts
- Shallow, thumbprint-like depressions on a meteorite's surface, sculpted by melting during its fiery descent.
- Rock
- A naturally bonded aggregate of one or more minerals; unlike a mineral, it has no fixed chemical formula.
- Rock cycle
- The continual transformation of rock among igneous, sedimentary, and metamorphic forms through melting, erosion, and burial.
S
- Schiller
- A bronzy or metallic internal sheen caused by light reflecting off tiny inclusions, seen in moonstone and sunstone.
- Sectile
- Able to be cut or sliced with a knife into shavings without crumbling, a tenacity property of minerals like gypsum.
- Sedimentary
- Rock formed from compacted and cemented sediment, often layered and the main home of fossils.
- Specific gravity
- A measure of a mineral's density compared to water; high values make minerals like galena feel unusually heavy.
- Stabilized
- Treated by infusing a porous stone with resin to harden it and deepen color, extremely common with turquoise.
- Streak
- The color of a mineral's powder, seen by rubbing it on unglazed porcelain — often steadier than surface color.
- Synthetic
- A lab-grown material with the same composition and structure as its natural counterpart — real, but not earth-formed.
T
- Tabular
- A crystal habit describing flat, plate-like or tablet-shaped crystals.
- Tenacity
- How a mineral resists breaking, bending, or crushing — described as brittle, malleable, sectile, or elastic.
- Thin section
- A wafer of rock ground so thin that light passes through it, letting geologists identify minerals under a microscope.
- Translucent
- Allowing some light to pass but not a clear image; between transparent and opaque.
- Treatment
- Any enhancement of a natural stone, such as heating, dyeing, irradiation, or coating, ideally disclosed by sellers.
- Triplet
- An assembled stone of three layers, such as opal with a dark backing and a clear protective cap.
- Tumbled
- Stones smoothed and polished by tumbling in abrasive grit, producing the rounded 'worry stones' sold in shops.
- Twinning
- The symmetric intergrowth of two or more crystals of the same mineral sharing a common structure.
U
- Ultramarine
- The intense blue pigment historically made by grinding lapis lazuli, once more costly than gold.
V
- Vein
- A sheet-like mineral deposit that fills a fracture in rock, often the source of ore and crystals.
- Vesicular
- Full of small holes left by gas bubbles trapped in cooling lava, as in pumice and vesicular basalt.
- Vitreous
- A glassy luster, the most common non-metallic luster, seen in quartz and many gems.
- Vug
- A small rock cavity, often lined with crystals — essentially a miniature pocket for crystal growth.
W
- Weathering
- The breakdown of rock at Earth's surface by water, air, temperature, and life, producing the sediment that later forms new rock.
- Widmanstätten pattern
- Interlocking crystal bands revealed by etching a cut iron meteorite, formed by cooling over millions of years and impossible to fake.
Z
- Zoning
- Variation in color or composition within a single crystal, such as the color-zoned tips of amethyst.