The crystal habit of a mineral describes its visible external shape. It can apply to an individual crystal or an assembly of crystals.
In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.
Factors influencing a crystal habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies like growth striations. Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the same mineral; corundum.
Some minerals may replace other existing minerals while preserving the original's habit: this process is called pseudomorphous replacement. A classic example is tiger's eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
The names of crystal habits are derived from:
Predominant crystal faces (prism - prismatic, pyramid - pyramidal and pinacoid - platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).
List of crystal habits
|Acicular||Needle-like, slender and/or tapered||Natrolite, Rutile|
|Amygdaloidal||Almond-shaped||Heulandite, subhedral Zircon|
|Bladed||Blade-like, slender and flattened||Actinolite, Kyanite|
|Botryoidal or globular||Grape-like, hemispherical masses||Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite|
|Columnar||Similar to fibrous: Long, slender prisms often with parallel growth||Calcite, Gypsum/Selenite|
|Coxcomb||Aggregated flaky or tabular crystals closely spaced.||Barite, Marcasite|
|Cubic||Cube shape||Pyrite, Galena, Halite|
|Dendritic or arborescent||Tree-like, branching in one or more direction from central point||Pyrolusite and other Mn-oxide minerals, Magnesite, native copper|
|Drusy or encrustation||Aggregate of minute crystals coating a surface or cavity||Uvarovite, Malachite, Azurite|
|Enantiomorphic||Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals||Quartz, Plagioclase, Staurolite|
|Equant, stout||Length, width, and breadth roughly equal||Olivine, Garnet|
|Fibrous||Extremely slender prisms||Serpentine group, Tremolite (i.e. Asbestos)|
|Filiform or capillary||Hair-like or thread-like, extremely fine||many Zeolites|
|Foliated or micaceous or lamellar (layered)||Layered structure, parting into thin sheets||Mica (Muscovite, Biotite, etc.)|
|Granular||Aggregates of anhedral crystals in matrix||Bornite, Scheelite|
|Hemimorphic||Doubly terminated crystal with two differently shaped ends.||Hemimorphite, Elbaite|
|Hexagonal||Hexagon shape, six-sided||Quartz, Hanksite|
|Hopper crystals||Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity||Halite, Calcite, synthetic Bismuth|
|Mamillary||Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates||Malachite, Hematite|
|Massive or compact||Shapeless, no distinctive external crystal shape||Limonite, Turquoise, Cinnabar, Realgar|
|Nodular or tuberose||Deposit of roughly spherical form with irregular protuberances||Chalcedony, various Geodes|
|Octahedral||Octahedron, eight-sided (two pyramids base to base)||Diamond, Magnetite|
|Plumose||Fine, feather-like scales||Aurichalcite, Boulangerite, Mottramite|
|Prismatic||Elongate, prism-like: crystal faces parallel to c-axis well-developed||Tourmaline, Beryl|
|Pseudo-hexagonal||Hexagonal appearance due to cyclic twinning||Aragonite, Chrysoberyl|
|Radiating or divergent||Radiating outward from a central point||Wavellite, Pyrite suns|
|Reniform or colloform||Similar to botryoidal/mamillary: intersecting kidney-shaped masses||Hematite, Pyrolusite, Greenockite|
|Reticulated||Crystals forming net-like intergrowths||Cerussite|
|Rosette or lenticular (lens shaped crystals)||Platy, radiating rose-like aggregate||Gypsum, Barite (i.e. Desert rose)|
|Stalactitic||Forming as stalactites or stalagmites; cylindrical or cone-shaped||Calcite, Goethite|
|Stellate||Star-like, radiating||Pyrophyllite, Aragonite|
|Striated||Not a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals||Tourmaline, Pyrite, Quartz, Feldspar, Sphalerite|
|Stubby or blocky or tabular||More elongated than equant, slightly longer than wide, flat tablet shaped||Feldspar, Topaz|
|Platy||Flat, tablet-shaped, prominent pinnacoid||Wulfenite|
|Tetrahedral||Tetrahedra-shaped crystals||Tetrahedrite, Spinel, Magnetite|
|Wheat sheaf||Aggregates resembling hand-reaped wheat sheaves||Stilbite|