Extremely large feldspar crystals are found in a coarse-grained igneous rock called pegmatite. Feldspars range in color from clear white or gray to shades of blue, green, or pink. All feldspars contain alumina and silica. Feldspars may be classified into two general groups, alkali feldspars and plagioclase feldspars, according to the other elements they contain.
All alkali feldspars contain potassium and most contain sodium.
The most common minerals in this group are microcline, orthoclase, and sanidine. Most plagioclase feldspars, such as andesine and labradorite, contain both sodium and calcium. Some feldspar crystals, called perthites, consist of alkali and plagioclase feldspars. Potassium feldspar Feldspar is used in making glass and ceramics. Feldspar crystals of especially beautiful color and luster may be used as gemstones, ornaments, and architectural decorations.