Guide To Crystal Terminology

Acicular

Long and slender needle shaped crystals

Adamantine

Lustre describing diamond like light reflecting properties

Aggregate

Composed of mineral crystals of one or more kind, or of mineral rock fragments, defined by the way they are clustered together

Alluvial

Material deposited by running water

Amorphous

Having no definite crystalline form

Adularescence

A pearly ghostlike bluish-white light reflection which glides over a mineral surface, caused by structural factors or a build up of water in the mineral. Opal and moonstone commonly display adularescence

Aphanitic

Igneous rock in which the crystalline structure or grain is too fine to be seen by the unaided eye

Arborescent

Aggregate composed of tree like or skeletal formations. Also known as dentritic

Asterism

Star-like light effect which glides over a mineral surface, caused by dense inclusions of thin parallel fibres within the mineral

Bipyramidal

Crystal shape dividing crystal into two pyramids

Birefringent

Light effect causing rays to separate into two and causing an observable doubling of an image as seen through the crystal. Also known as double refraction

Botryoidal

Aggregate clustered together and resembling a bunch of grapes

Brecciated

Clastic sedimentary rock composed of angular fragments cemented by various minerals, or an igneous rock formation resembling a breccia like texture

Carat

Weight measurement for gemstones equating to 200 milligrams. Abbreviated to Ct. Five Ct. = 1 gram

Chatoyancy

Concentrated narrow band light effect which glides over a mineral surface, caused by dense inclusions of thin parallel fibres within the mineral. Also known as Cats Eye

Clarity

Property of transmitted light with minimal scattering caused by inclusions and imperfections

Cleavage

The fracture, parting or splitting of a crystal along a definite crystalline plane resulting in a smooth surface plane

Cluster

A dense agglomeration of crystals

Coarse

Composed of relatively large particles

Columnar

Aggregate which has adjoining, compact, parallel and slender crystals

Compact

Dense, closely and firmly packed together

Conchoidal

Mineral fracture resembling a shell shaped indentation

Concretion

Aggregate composed of minerals cemented together in a rounded form

Concretionary

A rounded mass of minerals found in sedimentary rock

Cryptocrytalline

Mineral composed of microscopic crystals giving an amorphous appearance. Also known as microcrystalline

Crystal

A homogenous solid formed by a repeating, three dimensional pattern of atoms, ions or molecules and having a fixed distance between constituent parts

Crystalline

Has a crystal structure or compose of visible crystals

Cubic

Belonging to the isometric system, forming a six sided polyhedron

Cut

Definition of the surface form of a gemstone, can be natural or hand shaped or faceted

Density

The weight ratio of a mineral relative to an equal amount of water. Also known as specific gravity

Deposit

Accumulated minerals within a rock

Dichroic

Displaying two different colours when viewed from two different angles

Di-pyramidal

Synonym of bipyramidal

Disseminated

A hydrothermal ore deposit in which ore minerals are dispersed throughout a rock

Dodecahedron

Belonging to the isometric system, forming a twelve sided polyhedron

Double refraction

Light effect causing rays to separate into two and causing an observable doubling of an image as seen through the crystal. Also known as birefringent

Drusy

Aggregate composed of prismatic crystals protruding from a cavity

Dull

Lustre describing very poor optical properties

Earthy

Minerals which crumble under stress. Describes a type of fracture. Lustre describing poor reflective surface

Encrusting

Forming a disorganised mineral coating

Enhydro

Fluid inclusion, usually observable when air bubble present

Extrusive

Igneous volcanic rock formed by magma crystallising at the Earth’s surface

Feldspar

Minerals with similar characteristics and belonging to the Feldspar group, rock forming and abundant

Fibrous

Aggregate composed of fine parallel threads

Fluorescence

Light glow given off when exposed to ultraviolet light. Minerals may fluoresce different colours under different UV wavelengths

Foliated

Aggregate composed of thin leaf-shaped crystals

Fracture

Characteristic stress break mark not in a cleavage plane

Geode

Hollow rock containing crystals

Globular

Any rounded agglomeration, or synonym of botryoidal

Granular

Aggregate composed of grain shaped crystals

Greasy

Lustre describing a greasy appearance of a mineral

Habit

Attributes describing a crystals appearance

Hardness

Resistance to scratching by another substance. Diamond will scratch quartz and is therefore harder than quartz. Quartz will scratch selenite and is therefore harder than selenite

Hemimorphic

Double terminated crystal with different shaped ends

Hexagonal

Belonging to the hexagonal system, forming a six sided polyhedron with a top and bottom base

Hexoctahedron

A crystal composed of 48 triangular faces

Hydrothermal

Solutions of hot water arising from underwater sources

Igneous

Rock formed from volcanic action, can be amorphous or crystalline

Inclusion

Substance enclosed inside a crystal as it is forming

Incrustation

A disorganised mineral coating

Intrusive

Igneous volcanic rock formed by magma crystallising below the Earth’s surface

Iridescence

Display of colour light effect, similar to oil on water, from a mono-coloured mineral surface

Isometric

Belonging to the isometric system, describing a six sided polyhedron equal in all three dimensions

Labradorescence

A metallic blue, green, red or yellow light reflection which glides over a mineral surface, caused by structural factors in the mineral. Labradorite commonly displays Labradorescence

Lattice

The arrangement of atoms in a crystal

Lenticular

Flattened, lens-like shaped mineral, commonly concretions

Lustre

Properties describing the surface of a mineral including reflection

Macrocrystalline

Having crystals large enough to be seen unaided

Mafic

Dark coloured magnesium and iron bearing rocks

Magmatic

Synonym of igneous

Malleable

Ability to yield to hammering or rolling by thinning without cracking or breaking

Mammilary

Aggregate composed of smooth, rounded agglomerations

Massive

Mineral or rock that is non-crystalline, or composed of microscopic unorganised crystals or is a shapeless fragment

Matrix

A substance with a crystal emerging from or embedded within it

Metallic

Lustre describing an opaque reflective metal-like appearance

Metamorphic

Mineral environment where secondary minerals form from alteration through heat and pressure of the original

Micaceous

Aggregate composed of compact, flat, flexible, parallel and cleavable sheets

Microcrystalline

Mineral composed of microscopic crystals giving an amorphous appearance. Also known as cryptocrystalline

Mineral

A naturally occurring, homogeneous inorganic solid substance or element having a definite chemical composition and characteristic crystalline structure

Mineraloid

Amorphous and mineral in nature, for example obsidian

Moh’s Scale

A measurement scale to describe the hardness of a mineral

Monoclinic

Belonging to the monoclinic system, describing a six sided polyhedron unequal in all three dimensions. Two axis are at right angles to each other while the third lies at an angle other than 90°

Monozonitic

Rock formation

Nodular

Aggregate spherical in shape

Octahedron

Belonging to the isometric system, describing an eight sided polyhedron, where all sides are equal dimensional

Oolitic

Aggregate composed of very small spherical particles

Opalescence

Light effect exhibiting a play of colours when seen from different angles. Opals commonly display opalescence

Opaque

Property of transmitted light through a crystal. Opaque crystals are not able to transmit light

Orthorhombic

Belonging to the orthorhombic system, describing a six sided polyhedron unequal in all three dimensions with all axis at 90°

Oxidation

Chemical alteration through exposure to oxygen

Paramagnetic

Weak attraction to a magnetic field

Pearly

Lustre describing a similarity to the inside of a shell, may have iridescent hue

Pegmatite

Igneous rock formation with large individual grains and crystals

Phosphorescence

Light glow given off when exposed to ultraviolet light and continuing after the UV source has been removed

Piezoelectric

Ability to generate electrical charge when placed under stress

Pinacoidal

Elongated crystal

Pisolitic

Aggregate composted of small distorted spherical particles larger than oolitic minerals

Platy

Small, flat and flaky

Pleochroism

Displaying two or more different colours when viewed from two different angles

Polymorph

Chemically equivalent minerals with differing atomic crystal lattice arrangements

Porous

Containing small or microscopic holes throughout

Porphyritic

Igneous rock in which the crystalline structure or grain is large enough to be seen by the unaided eye

Prismatic

Crystal with uniform cross section and usually elongated in one direction

Pyritohedron

Belonging to the isometric system, describing a twelve sided polyhedron, where all sides pentagonal are equal dimensional

Pyroelectric

Ability to generate electrical charge during a change in temperature

Radiating

Aggregate composed of slender compacted crystals radiating from a central point in two or three dimensions

Reniform

Aggregate composed of smooth kidney shaped agglomerations

Resinous

Lustre describing yellow to brown minerals with high brilliance

Reticulated

Aggregate composed of long crystals crisscrossing each other

Rhombohedron

Six sided polyhedron with each side shaped as a rhombus

Rock

A naturally occurring aggregate of minerals and or mineraloids

Rosette

Mineral with concentric rose-like shaped aggregates

Scalenohedron

Six sided polyhedron, bipyramidal with a diagonal between every side at the centre

Schiller

Colour flashes reflected from a mineral

Schist

Metamorphic rock composed of parallel layers of micaceous minerals

Sedimentary

Rock formed by weathering forming layers from the accumulation of minerals and organic substances

Silicified

Organic material replaced by silica

Silky

Lustre describing very fine fibrous mineral giving optical properties similar to silk

Sphenoidal

Prism with wedge shaped corners

Stalactitic

Aggregate composed of long, icicle-like shaped formations

Striated

Exhibiting parallel grooves on the crystal face

Stubby

Short and fat

Sub-metallic

Lustre describing opaque minerals with good reflective properties

Tabular

Crystal habit describing flat four sided crystal

Tetragonal

Belonging to the tetragonal system, describing a six sided polyhedron where two axis are equal in length, one unequal, and with all axis at 90°

Tetrahedron

Belonging to the isometric system, describing a four sided polyhedron where all sides are equal dimensional and triangular

Thermoluminescence

Light glow given off when exposed to heat

Translucent

Property of transmitted light through a crystal. Translucent crystals are able to transmit light but not fully, such that objects appear unclear observed through the crystal

Transparent

Property of transmitted light through a crystal. Transparent crystals are fully able to transmit light

Trapezohedron

Belonging to the isometric system, describing a 24 sided polyhedron, all sides equal dimensional

Triboluminescence

Brief orange light glow given off when exposed to stress such as being struck

Triclinic

Belonging to the triclinic system, describing a six sided polyhedron, three unequal axes, none at right angles to each other

Trigonal

Belonging to the trigonal system, part of the hexagonal system, describing a six sided polyhedron with three axis equal in length, at 120° from each other, and a fourth is longer or shorter and at right angles to the others

Trillings

Intergrowth of three orthorhombic crystals that twin, forming a hexagonal shaped crystal at the centre

Vitreous

Lustre describing minerals with reflective properties similar to glass

Waxy

Lustre describing a waxy like appearance of a mineral