Diamond. This mineral is remarkable not only because it is the king of precious stones, but also because it is easily the hardest known substance (10 in the scale). Specific gravity, 3.5. Very brilliant luster. Crystals of usually octahedral habit in the isometric system. Usually colorless, but often variously tinted. Composition, pure carbon. Burns completely away at high temperature. The greatest mines in the world are in South Africa, where the diamonds occur in masses of rather soft (decomposed) igneous rock, evidently having crystallized during the cooling of the molten masses. In Brazil and India diamonds are found in stream gravels. Feldspar Group. The feldspars are by far the most abundant of all minerals in the crust of the earth. (Figures 74c, 74d.) There are several important species or varieties of feldspar with certain features in common as follows: crystal forms, either monoclinic or triclinic (closely resembling monoclinic), in prismatic forms whose faces usually meet at or near 90 or 120 degrees; two good cleavages at or near 90 degrees, hardness at or near 6; specific gravity, a little over 2.5; color, usually white, gray, or pink; and composition, silicate of aluminum with potash, soda, or lime. The two potash feldspars are _orthoclase_ and _microcline_, the former being monoclinic, with cleavages at exactly 90 degrees, and the latter triclinic, with cleavages a little less than 90 degrees. A kind of green microcline is known as _Amazon stone_. The soda-lime feldspars go by the general name _plagioclase_. They are triclinic, with cleavages meeting at approximately 86 degrees. Very commonly one of the cleavage faces exhibits characteristic, well defined striations or fine parallel lines caused by multiple twinning during crystal growth. Some of the common plagioclases are _albite_, a white soda feldspar, including most so-called _moonstone_; _oligoclase_, a usually greenish-white to reddish-gray soda-lime feldspar including _sunstone_; and _labradorite_, a lime-soda feldspar, usually gray to greenish-gray with a beautiful play of colors. The feldspars occur in all three great groups of rocks, but they have most commonly crystallized during the cooling of molten masses of igneous rocks. Where many sedimentary rocks have undergone great change (metamorphism) under conditions of heat, pressure, and moisture, feldspars have very commonly formed. Orthoclase and microcline feldspar are used in the manufacture of porcelain and chinaware. Some special varieties of feldspar are cut or polished for semiprecious stones or decorative purposes.