Feldspar is an important ingredient in the manufacture of glass. Feldspar is the most common rock-forming mineral. Feldspar is a name for a group of minerals with a general chemical formula of x Al(Al,Si)₃O₈, where x can be one or more elements. Feldspar minerals are usually white or very light in color and have a hardness of 6 on the Mohs’ Scale of Hardness. Another physical property of the feldspar group is that the mineral has good cleavage in two directions. Feldspar minerals break down to form kaolinite, a clay mineral.