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.