Allotropy, uh LOT ruh pee, in chemistry, is the ability of an element to exist in more than one form. These forms are called allotropes. For example, three common allotropes of carbon are: (1) hard, transparent diamond crystals, (2) soft, steel-gray or black graphite crystals, and (3) black, sooty, uncrystallized carbon black (see Carbon). The word allotropy comes from two Greek words meaning another and way.