Click on a letter above to view the list of gems.    

Chrystallography of Gems

Crystal molecules grow in three dimensional patterns or arrays. The atoms and molecules are locked in periodic or repeated arrays. These arrays can be categorized into six major crystal systems: hexagonal, isometric, monoclinic, orthorhombic, tetragonal, and triclinic. Trigonal has been called a seventh crystal system, but is actually only a subclass of the hexagonal crystal system and not a separate crystal system.

Each of these six major crystal systems is defined in terms of crystal axes, which are imaginary lines in space that intersect at a common point. The lengths of these axes can be described as equal or unequal to each other and further described in terms of the angles that the axes make with each other. These crystal systems may affect the external characteristics of a mineral such as crystal shape (habit).
 

Triclinic
Three unequal axes all intersecting at oblique angles.

Minerals that crystallize in the triclinic crystal system include Axinite and Plagioclase.

Monoclinic
Three unequal axes, two of which are inclined to each other at an oblique angle and the third perpendicular to the plane of the other two.

Minerals that crystallize in the monoclinic crystal system include Pyroxene, Amphibole, Orthoclase, Azurite and Malachite among many others.

Orthorhombic
Three mutually perpendicular axes all of different lengths.

Minerals that crystallize in the orthorhombic crystal system include Olivine (Peridot) and Barite.

Tetragonal
Three mutually perpendicular axes, two of which (the horizontal axes) are of equal length, but the vertical axis is shorter or longer than the other two.

Minerals that crystallize in the tetragonal crystal system include Zircon and Cassiterite.

Hexagonal
Three equal horizontal axes intersect at angles of 120 degrees, the fourth (vertical) is of different length and perpendicular to the plane of the other three. The hexagonal crystal system is divided into the hexagonal and rhombohedral or trigonal divisions.

Minerals that crystallize in the hexagonal division include apatite, beryl and high quartz. Minerals that crystallize in the rhombohedral division are Calcite, Dolomite, low Quartz and Tourmaline.

Isometric (Cubic)
Three mutually perpendicular axes of equal lengths. Crystal forms in the isometric system have the highest degree of symmetry when compared to all the other crystal systems.

Minerals that crystallize in the isometric (cubic) crystal system include Cuprite, Diamond, Fluorite, Gahnite, Garnet, Hackmanite, Hauyne, Lazurite, Leucite, Rhodizite, Sphalerite, Spinel and Villiaumite.

The six crystal systems describe the patterns of the molecules of crystals. Crystals can also be described by their external shape or "habit". Terms for crystal shapes include acicular (needlelike), bladed (flat with sharp edges), equant (roughly equal length sides), filiform (hairlike), prismatic (elongated), pyramidal (shaped like single or double pyramids) and tabular (flat with flat, perpendicular edges).

The way minerals grow is greatly affected by the environment and temperature in which they form. Minerals deposited in sedimentary environments are usually earthy (masses of densley packed powder), stalactitic (shaped lick stalactites), oolitic or (masses of spherical grains) and sometimes massive (solid and chunky). Igneous minerals are usually crystalline (varioius crystal shapes), massive (solid and chunky) and sometimes cleavable (crystalline masses that can be cleaved).

Below is a more detailed list of crystal habits.
 

List of Crystal Habits (shapes)

Habit

Description

Example

Acicular

Needle-like, slender and/or tapered

Rutile in Quartz

Amygdaloidal

Almond-shaped

Heulandite

Anhedral

Poorly formed, distorted

Olivine

Bladed

Blade-like, slender and flattened

Kyanite

Botryoidal or globular

Grape-like, hemispherical masses

Smithsonite

Columnar

Similar to fibrous: Long, slender prisms often with parallel growth

Calcite

Coxcomb

Aggregated flaky or tabular crystals closely spaced.

Barite

Dendritic or arborescent

Tree-like, branching in one or more direction from central point

Magnesite in Opal

Dodecahedral

Dodecahedron, 12-sided

Garnet

Drusy or encrustation

Aggregate of minute crystals coating a surface

Uvarovite

Enantiomorphic

Mirror-image habit and optical characteristics; right- and left-handed crystals

Quartz

Equant, stout, stubby or blocky

Squashed, pinnacoids dominant over prisms

Zircon

Euhedral

Well-formed, undistorted

Spinel

Fibrous or columnar

Extremely slender prisms

Tremolite

Filiform or capillary

Hair-like or thread-like, extremely fine

Natrolite

Foliated or micaceous

Layered structure, parting into thin sheets

Mica

Granular

Aggregates of anhedral crystals in matrix

Scheelite

Hemimorphic

Doubly terminated crystal with two differently shaped ends.

Hemimorphite

Mamillary

Breast-like: intersecting large rounded contours

Malachite

Massive or compact

Shapeless, no distinctive external crystal shape

Serpentine

Nodular or tuberose

Deposit of roughly spherical form with irregular protuberances

Geodes

Octahedral

Octahedron, eight-sided (two pyramids base to base)

Magnetite

Plumose

Fine, feather-like scales

Mattramite

Prismatic

Elongate, prism-like: all crystal faces parallel to c-axis

Tourmaline

Pseudo-hexagonal

Ostensibly hexagonal due to cyclic twinning

Aragonite

Pseudomorphous

Occurring in the shape of another mineral through pseudomorphous replacement

Tiger's eye

Radiating or divergent

Radiating outward from a central point

Pyrite suns

Reniform or colloform

Similar to mamillary: intersecting kidney-shaped masses

Hematite

Reticulated

Acicular crystals forming net-like intergrowths

Cerussite

Rosette

Platy, radiating rose-like aggregate

Gypsum

Sphenoid

Wedge-shaped

Sphene

Stalactitic

Forming as stalactites or stalagmites; cylindrical or cone-shaped

Rhodochrosite (Argentina)

Stellate

Star-like, radiating

Pyrophyllite

Striated/striations

Surface growth lines parallel or perpendicular to c-axis

Chrysoberyl

Tabular or lamellar

Flat, tablet-shaped, prominent pinnacoid

Ruby

Wheat sheaf

Aggregates resembling hand-reaped wheat sheaves

Zeolites