Crystal habit

The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to limited space in the crystallizing medium (commonly in rocks).

[1] Most natural crystals, however, do not display ideal habits and are commonly malformed.

Minerals belonging to the same crystal system do not necessarily exhibit the same habit.

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.

Smoky quartz with spessartine on top of feldspar matrix, featuring different crystal habits (shapes)
Goethite replacing pyrite cubes.