A common mineral occurring in monoclinic crystals, with a
   prismatic angle of nearly 90¡, and also in massive forms which are
   often laminated. It varies in color from white to dark green and black,
   and includes many varieties differing in color and composition, as
   diopside, malacolite, salite, coccolite, augite, etc. They are all
   silicates of lime and magnesia with sometimes alumina and iron.
   Pyroxene is an essential constituent of many rocks, especially basic
   igneous rocks, as basalt, gabbro, etc.