Factors influencing 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 such as 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 originals habit, i.e. pseudomorphous replacement. A classic example is tigers eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic (elongate, prism-like) crystals, in tigers eye the original fibrous' habit of crocidolite is preserved.