Crystalline Growth: Crystal Habits

Crystal Habit

The term "crystal habit" is used to categorize the appearance, shape, and size of a crystal, and identify its unique growth characteristics that result from its crystalline structure and growth environment.

Describing a mineral's crystal habit is useful in communicating what a certain specimen would, or should look like. There are approximately thirty-six standardized terms to describe the variations, or habits of a crystal's growth. A particular mineral may exhibit several different habits, all of which are influenced by the following factors:

1. Crystal Twinning ( two individual crystals share some of the same crystal lattice points)
2. Growth Conditions (heat, pressure, and space)
3. Trace Impurities (present during crystal formation)

Most gemstone crystals are not found as a perfectly formed single crystal, but are found as aggregates of several crystals that have grown together into a single mass. When one mineral replaces the void that is left behind from another mineral's disintegration, while preserving that original mineral's growth habit, it is referred to as a "pseudomorphous replacement."

Gemstones and their Crystal Habits

Crystal Habit:	Habit Description:	Mineral:
Acicular	Needle-like, slender and/or tapered	Rutile (Quartz)
Anhedral	Poorly formed and distorted	Olivine
Banded	narrow bands of differing colors	Onyx
Bladed	Slender, flattened and blade-like	Tanzanite
Columnar	Long, slender prisms and parallel growth	Calcite
Dendritic	Tree-like multi-directional branching from central point	Opal
Dodecahedral	12-sided Dodecahedron	Garnet
Drusy (Encrustation)	Aggregate of minute crystals coating a surface	Drusy (Quartz)
Enantiomorphic	Mirror-image (left/right) habit and optical characteristics	Citrine, Amethyst
Equant (Stubby)	Squashed, pinnacoids dominant over prisms	Zircon
Euhedral	Well-formed and Undistorted	Spinel
Foliated	Easily separated into plates	Mica
Geode	Rock cavities with internal crystals & encrustation	Amethyst, Quartz
Granular Mass	Anhedral crystals in lumpy mass with no crystal form	Peridot
Mamillary	Intersecting large rounded contours	Malachite
Massive	Shapeless with no distinct external crystal shape	Lapis Lazuli
Octahedral	Eight-sided octahedron - two pyramids, base to base	Diamond
Pincoid	Terminated by multiple pyramidal faces	Emerald, Beryl
Prismatic	Elongate and Prism-Like, all faces are parallel to c-axis	Tourmaline, Topaz
Pseudomorphous	Occurring in the shape of another mineral	Tiger's Eye
Striated	Surface growth lines parallel or perpendicular to c-axis	Alexandrite
Tabular (Lamellar)	Flat, tablet-shaped, prominent pinnacoid	Ruby