Ever wonder what botryoidal means or what exactly thumbnail size is? Then you've come to the right place.
Expand each section by clicking the + next to the section title to see all the terms. Click the - sign to condense and move on to a different section.
These are terms that you may come across in descriptions of specimens for sale.
Contact
A flattened or incomplete crystal face caused by crowding in growth.
Floater
A crystal fully developed on all sides with no attachment to the matrix.
Locality
The place where a specific mineral specimen was collected. Some localities are known for specific minerals.
Matrix
The host rock in which a mineral is embedded.
Old Stock
Material collected decades ago, often from closed or depleted localities.
Provenance
The documented history of ownership and origin of a specimen.
Pseudomorph (ps. / after)
A mineral that has replaced another but preserved the original’s external shape (e.g., quartz after fluorite).
Self-Collected
Collected from nature by the seller or dealer themselves.
Termination
The natural end of a crystal, often flat or sloping. When both terminations are intact it's referred to as double terminated.
Type Locality
The location where a mineral species was first discovered and described.
Variety (var.)
A form of a mineral that differs in color or habit but not in chemical formula (e.g., amethyst = var. of quartz).
Specimen Sizes
Since these are related, I'm presenting them as a group, in order from largest to smallest.
Cabinet
A large specimen for display, larger than miniature. You may also see terms like small cabinet and large cabinet.
Miniature
A specimen fitting in a 2″ box. Can usually fit in the palm of your hand.
Thumbnail
A specimen small enough to fit in a 1.25” box. Often kept in a specific box called a perky box.
Micromount
A very small mineral specimen (often <1 cm) that usually needs magnification to see well.
These terms describe physical properties that are often used in mineral identification.
Cleavage
The tendency of a mineral to break along planes of weakness.
Fracture
How a mineral breaks when it does not follow cleavage planes.
Hardness (Mohs Scale)
A relative scale (1–10) of scratch resistance, with talc = 1 and diamond = 10.
Luster
The way light reflects off a mineral’s surface (metallic, vitreous, silky, etc.).
Magnetism
Attraction to a magnet, typical of magnetite and pyrrhotite.
Specific Gravity
The density of a mineral compared to the density of water.
Streak
The color of a mineral’s powder when rubbed on unglazed porcelain.
Tenacity
A mineral’s resistance to breaking, bending, or crushing (e.g., brittle, malleable, flexible).
These are terms related to the visible shape of a mineral specimen. Though related to crystal systems and underlying structure, these can vary greatly for a single mineral.
Acicular
Needle‑like crystal habit (e.g., natrolite, scolecite).
Bladed
Thin, elongated crystals resembling knife blades (e.g., kyanite).
Botryoidal
Rounded, grape‑like formations, seen in hematite, smithsonite, and malachite.
Crystal Habit
The general external shape of a crystal (e.g., cubic, prismatic, acicular).
Druse / Druzy
A coating of tiny crystals lining a rock surface or cavity.
Euhedral
A crystal with well‑formed, easily recognizable external faces.
Geode
A rounded rock cavity lined with crystals, formed from mineral deposition inside voids.
Massive
A form where a mineral shows no visible crystals, occurring as a solid lump.
Prismatic Habit
Elongated crystals with well‑developed prism faces, like in tourmaline or emerald.
Stalactitic Form
Icicle‑like growths formed as minerals precipitate from dripping fluids. Stalagmites are similar but form from the ground up.
Tabular
Flat, plate‑like crystals (e.g., barite, wulfenite).
Twinning
Two or more crystals of the same mineral intergrown in symmetrical orientations. Can affect overall crystal shape.
Vug
A small cavity in a rock, often lined with crystals.
Every mineral belongs to one crystal system reflecting its underlying structure. Though a minerals crystal system affects its habit, these relationships are not always clear cut. These are not in alphabetical order, but in order of symmetry from most to least symmetrical.
Cubic (Isometric)
Three equal axes at 90°. Forms cubes, octahedra, dodecahedra (e.g., fluorite, pyrite).
Tetragonal
Two equal axes and one longer/shorter, all at 90°. Often prismatic (e.g., zircon).
Trigonal / Hexagonal
Four axes: three equal in a 120° plane, plus one vertical axis (e.g., beryl, quartz, calcite). Hexagonal is often considered a subset of trigonal.
Orthorhombic
Three unequal axes, all at 90°. Prismatic or rhombic crystals (e.g., topaz, olivine).
Monoclinic
Three unequal axes; two meet at 90°, the third oblique. Bladed or prismatic (e.g., gypsum).
Triclinic
Three unequal axes, none at 90°. Distorted or asymmetrical (e.g., kyanite, albite).
These are terms describing the very fun and incredible visual effects different minerals may exhibit.
Adularescence
A soft glow seen in moonstone due to light scattering.
Asterism
A star‑like optical effect in cabochon‑cut stones, caused by aligned inclusions.
Chatoyancy
The “cat’s eye” effect caused by fibrous inclusions or parallel structures (e.g., tiger’s eye).
Fluorescence
The emission of visible light by a mineral when exposed to ultraviolet (UV) light.
Labradorescence
The unique iridescence seen in labradorite.
Inclusion
A material (solid, liquid, or gas) trapped inside a mineral as it forms.
Iridescence
Rainbow‑like colors caused by light interference in thin layers or cracks.
Phosphorescence
A glow that continues briefly after the UV light is turned off.
Play of color
Iridescent flashes seen in minerals like opal.
Pleochroism
The property of some minerals to appear different colors when viewed from different directions.
Schiller effect
A metallic sheen caused by light scattering from microscopic inclusions. Seen in sunstone, moonstone, aventurine, and labradorite.
Translucence
Partial passage of light through a mineral; between transparent and opaque.
Zoning
Visible color bands in a crystal, caused by chemical changes during growth.
These terms are not essential to know if you're a collector, but you may come across them. I'm not a geologist, so I am still learning about these terms.
Amygdaloid
Basaltic rock where vesicles are filled with minerals.
Breccia
A rock composed of angular fragments cemented together.
Group
A family of related minerals with similar structures (e.g., feldspar or garnet group).
Hydrothermal Vein
Mineral deposit formed from hot, mineral‑rich fluids.
Oxidation Zone
Near‑surface part of a mineral deposit where sulfides alter to oxides and carbonates, often producing colorful minerals.
Pegmatite
Very coarse‑grained igneous rock, often hosting large gem crystals.
Polymorph (Dimorph / Trimorph)
Minerals with the same chemistry but different structures (e.g., calcite vs. aragonite; kyanite, andalusite, sillimanite).
Series
A continuous range of solid solutions between two mineral species (e.g., calcite-rhodochrosite series).
Species
A distinct mineral with a unique chemical composition and crystal structure.
Vesicle
A gas bubble cavity in volcanic rock, often later filled with minerals (forming amygdales).
