the ability of grains of minerals during their crystallization in rocks or ore bodies to assume the form of well-faceted crystals typical of these minerals. Minerals that crystallize in the early stage of the formation of a rock or ore from astatic media (for example, melts or water solutions) usually have the most marked idiomorphism. The degree of idio-morphism is also determined by the crystallizing capability of the substance, and for this reason minerals that recrystallize in a solid medium or that have formed in the late stages of the crystallization of the minerals of a rock or ore body can also be idiomorphic. Common idiomorphic minerals in rocks include feldspars, augites, amphiboles, quartz, and nepheline; among the ores, pyrite and arsenopyrite are often idiomorphic.