Feldspars are the most abundant and widespread minerals of the crust, or outermost part, of the Earth. Because of their abundance, feldspars are used in the classification of IGNEOUS ROCKS. They are also abundant in METAMORPHIC ROCKS and in some sediments and SEDIMENTARY ROCKS, especially those formed in arid and semiarid regions. Feldspars are also major constituents of moon rocks, which are similar to rocks of the Earth's crust. Feldspars are used in the manufacture of ceramics and ceramic glazes and as mild abrasives. A few varieties, including labradorite and orthoclase, are occasionally used as gems because they show an iridescent play of colors or a milky translucence. Milky varieties are termed moonstone. Feldspars are aluminosilicates, containing silicon and aluminum ions bound together by oxygen ions to form a three-dimensional framework of atoms. Other ions, principally potassium, sodium, and calcium, occupy sites within the framework and give rise to the three pure, or end-member species: orthoclase, albite, and anorthite. Feldspars are slightly translucent and have a glassy, or vitreous, luster rather like that of glazed porcelain. Because their atomic framework has planes of weakness, feldspars exhibit good cleavage, breaking readily into blocky pieces with smooth sides. On the Mohs mineral hardness scale, feldspars are 6--slightly harder than a steel knife blade and about as hard as porcelain.