A name given to a group of minerals, closely related in
   crystalline form, and all silicates of alumina with either potash,
   soda, lime, or, in one case, baryta. They occur in crystals and
   crystalline masses, vitreous in luster, and breaking rather easily in
   two directions at right angles to each other, or nearly so. The colors
   are usually white or nearly white, flesh-red, bluish, or greenish.