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Crystal and Mineral Terms: A Collector's Glossary

By Digital Towns Crystals ยท Last reviewed October 8, 2026

Short answer: most confusion in crystal listings comes from about forty words. "Matrix" is the host rock a crystal grew on; a "floater" grew with no attachment point; a "vug" is a crystal-lined pocket that stays part of its rock; "druse" is a crust of tiny points; "habit" is a mineral's usual shape; "streak" is the color of its powder. The definitions below are grouped by what they describe, each with a link to the deeper guide.

What separates a mineral, a rock and a mineraloid?

Mineral. The working geologist's test has five parts. Geology.com lists them as naturally occurring, inorganic, solid, a definite chemical composition and an ordered internal structure. Steel fails the first test, pearl fails the second. The edge cases are instructive: although liquid water is not a mineral, ice qualifies once it freezes.

Rock. The USGS defines a rock as an aggregate of one or more minerals, or a body of undifferentiated mineral matter. Granite, basalt and limestone are rocks; the quartz and feldspar grains inside granite are minerals.

Mineraloid. A naturally occurring solid that has no crystal lattice. Minerals.net gives opal as its example. Obsidian, amber and moldavite sit in the same bucket; the mineraloids guide explains why that matters for hardness and fracture.

Polymorph. Same chemistry, different atomic arrangement. The textbook pair is diamond and graphite, both pure carbon (Minerals.net). Calcite and aragonite are the pair crystal buyers meet most often.

Variety. A color or texture version of one mineral species that carries its own trade name: amethyst, citrine and rose quartz are all quartz. Names that sound like species but are not are covered in the trade names guide.

Crystal system. The symmetry family a mineral's lattice belongs to. Minerals.net groups crystals into six systems (isometric, tetragonal, hexagonal including trigonal, orthorhombic, monoclinic, triclinic) and notes the finer division into 32 crystal classes. The crystal systems guide walks through each with examples, and the planned mineral classes guide sorts minerals by chemistry instead.

What do the words for crystal shape mean?

Habit. The characteristic outward look of a crystal or a group of crystals: bladed, prismatic, cubic, needle-like and so on. Kyanite is bladed, pyrite often cubic. See crystal habits.

Euhedral. Bounded by sharp, well-developed faces. A euhedral crystal had room to grow freely; an anhedral grain was crowded by its neighbors and shows no proper faces.

Massive. No crystal form visible to the unaided eye. Rose quartz is almost always massive, which is why it is sold as spheres and tumbles rather than points.

Botryoidal. Rounded bubbles like a bunch of grapes. Minerals.net ranks the sizes of these rounded habits: botryoidal clusters are smaller than reniform (kidney) masses and much smaller than mammillary ones. Malachite and hematite are common botryoidal examples.

Dendrite. A branching, fern-like growth, usually of manganese or iron oxide along a crack. Not a fossil plant, despite the look. See dendrites.

Termination. The pointed or faceted end of a crystal, as opposed to the broken base where it was attached. Doubly terminated means both ends are complete; Herkimer diamonds are the best-known example, and the double-terminated crystals guide covers how they form.

Striations. Fine parallel grooves on a crystal face. Horizontal striations across quartz prism faces and lengthwise striations on tourmaline are useful identification clues.

Twin. Two or more crystals of the same mineral grown together according to a fixed geometric law, such as a Japan law quartz twin or a fishtail gypsum twin. See twinning.

How is a specimen put together?

Matrix. The rock or mineral a crystal grew on and is still attached to. "On matrix" in a listing means the crystals were not snapped off their host, which usually adds value and context. A dedicated matrix specimens guide is planned.

Floater. A crystal that grew suspended, often in clay, with no attachment point, so it is complete on all sides. Minerals.net also notes that dealers use the word loosely for any crystal that lacks matrix, so ask whether a "floater" grew that way or was simply detached.

Cluster. A dense group of crystals sharing one base, such as a quartz or apophyllite plate. See clusters.

Druse (drusy, druzy). A crust of many small, sparkling crystal points coating a surface or lining a cavity. See druzy.

Vug. A cavity in rock that may hold crystals. Geology.com's geode article draws the useful line: a vug lacks a competent lining, so it will not survive as a separate object once its host rock weathers away. The famous doubly terminated Herkimer quartz grows in vugs in the Little Falls Dolostone of Herkimer County, New York.

Geode. A hollow, or once hollow, rounded body with a tough outer shell that lets it separate intact from the surrounding rock. Linings are typically chalcedony and quartz, sometimes amethyst or calcite. The same article records a Marrakesh "geode" made of pottery, with galena crystals glued on to imitate a druse, so a perfect-looking cheap geode deserves a close look. See geodes.

Associated species. Other minerals sharing the specimen, such as red mordenite with stilbite. They are often the best clue to where a piece came from.

What do the physical property terms mean?

Hardness. Resistance to scratching, ranked on the Mohs scale from talc at 1 to diamond at 10. See hardness testing and the planned hardness chart.

Cleavage. A tendency to split along flat planes set by the lattice. Calcite breaks into rhombs; mica peels into sheets.

Fracture, conchoidal. Breakage that does not follow planes. Conchoidal fracture leaves curved, shell-like scoops, as on obsidian and quartz.

Luster. How a surface reflects light. Geology.com's luster guide says about 70% of all minerals can show vitreous (glassy) luster; other terms include metallic, pearly, greasy, silky, resinous and adamantine.

Streak. The color of a mineral's powder, made by dragging it across unglazed white porcelain. It is steadier than surface color: per Geology.com's streak test, all hematite leaves a reddish streak whatever its outer color, brassy pyrite leaves a black streak, and fluorite gives a white streak in every color it comes in. That is the quickest separation of hematite from look-alikes.

Specific gravity (SG). A mineral's weight compared with an equal volume of water. Quartz runs 2.6 to 2.7 (Geology.com quartz data); a "quartz" that feels heavy for its size deserves a specific gravity test.

Which terms describe optical effects?

Fluorescence. A glow under ultraviolet light that stops when the lamp goes off. Phosphorescence is the rarer case where the glow lingers for a brief time afterward (Geology.com). Collectors' lamps are sold by wavelength: shortwave at 255 nm, midwave at 310 nm and longwave at 365 nm. Many minerals glow one color under shortwave and another under longwave. See fluorescence and UV testing.

Chatoyancy. A single bright band of light gliding across a polished cabochon, the cat's eye effect, caused by parallel fibers or tubes. Tiger's eye is the everyday example. See chatoyancy.

Asterism. A star of light on a cabochon, from inclusions arranged in more than one direction. Stars show four, six or occasionally twelve rays (Minerals.net).

Iridescence, labradorescence, adularescence, play of color. GIA's guide to phenomenal gems treats these as separate effects: the rainbow sheen of fire obsidian from nanoscale magnetite layers, the blue to gold flash of labradorite, the floating glow of moonstone and the patchwork of spectral color in opal. See iridescence.

Schiller. An older umbrella word for a colored flash or sheen; minerals.net also applies it to the individual color patches in opal.

Inclusion. Anything trapped inside a crystal while it grew: needles of rutile, flakes of mica, liquid and gas bubbles. Phantom is a special case, a ghost outline of an earlier crystal face preserved inside a later one. See inclusions and phantom quartz.

Pseudomorph. One mineral that has taken the outward shape of another, such as goethite in pyrite cubes. An epimorph is the variant where the new mineral coats the old one and the original then dissolves, leaving a hollow cast. See pseudomorphs.

What do treatment and finishing words mean?

Heat treatment. Exposing a stone to high temperature to change its color or clarity, as GIA's gem treatment overview defines it. Geology.com notes that much of the yellow quartz sold as citrine is amethyst that has been heated. See heat-treated crystals.

Irradiation. Exposure to an artificial radiation source to alter color, sometimes followed by heating, which GIA calls a combination treatment. See irradiated crystals.

Dyed. Color pushed into a porous or cracked stone; GIA notes cracks are sometimes deliberately induced by heating so the dye can enter. See dyed stones.

Stabilized and reconstituted. Resin soaked into soft material, or crushed material bonded back into a block. See stabilized and reconstituted stones.

Coated. A surface film, such as the metal layer on aura quartz.

Rough, tumbled, cabochon. Rough is uncut, unpolished material. Tumbled stones have been rolled in a barrel with grit until smooth, a process rivers also perform naturally. A cabochon is polished with a domed, unfaceted top. See tumbled stones and raw vs polished.

Loupe. A pocket magnifier. The International Gem Society calls 10X the standard for hand-held gem identification, because above that the depth of field becomes too shallow to use easily; a triplet loupe has three lenses to cut edge distortion. See using a loupe.

What do specimen size labels mean?

Micromount. A tiny specimen, about half an inch (13 mm) or smaller, mounted in a small box and studied under magnification (Minerals.net).

Thumbnail. A specimen about 1 inch (2.5 cm), traditionally kept in a small clear box (Minerals.net).

Miniature, small cabinet, cabinet. Progressively larger display sizes. Definitions vary by club and dealer, so check the measurements in the listing; the specimen sizes guide compares the conventions.

Reading a listing with these words: shopping examples we stock

The test of a glossary is whether it helps decode a real product title. When you buy, check that each technical word in the title is backed by the photos: "on matrix" should show a visible host rock, "phantoms" should be visible through a clear window, a size label should come with millimeters or grams. A specimen listing guide covers the rest.

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Frequently asked questions

What does "on matrix" mean for a crystal?

It means the crystal is still attached to the rock or mineral it grew on. Matrix pieces show how the crystal formed and are harder to fake convincingly than loose points, so collectors usually pay more for them. Check the photos for a natural attachment rather than a glued joint.

What is the difference between a vug and a geode?

Both are cavities that can hold crystals. A geode has a tough outer shell that lets it come free from its host rock as a round object. A vug has no such shell and stays part of the rock, so it is collected by breaking the rock open.

What is a floater crystal?

Strictly, a crystal that grew without touching any surface, often in soft clay, so it is complete on every side and has no broken base. Some sellers also use the word for any loose crystal without matrix, so ask how it formed.

Is a mineraloid a real mineral?

Not by the strict definition, because it lacks an ordered crystal lattice. Opal, obsidian and amber are mineraloids. Many references and shops still file them alongside minerals, and they are just as natural; the label only describes their internal structure.

Why does streak matter more than color?

Surface color varies with impurities and weathering, but the color of the powdered mineral stays consistent. Hematite can look black, silver or red and still always leaves a reddish streak, which makes streak one of the quickest identification checks on rough material.

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