Crystal habit
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In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals.
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognising 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 which are influenced by certain factors. Crystal habit may mislead the inexperienced as a mineral's crystal system can be hidden or disguised.
Factors influencing a crystal's habit include: a combination of two or more forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space). 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 euhedral (well-formed), prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
List of crystal habits
| 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 | Mottramite |
| 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 |
| 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 |