Diamond, composed of carbon, is the hardest natural substance in the world. Each carbon atom is surrounded by four neighboring carbon atoms in a tetrahedral coordination that is the result of a covalent bond and a face-centered arrangement in the cubic unit cell. Diamond is in the isometric crystal system, which is reflected in the commonly found octahedral or cubic crystal form. The external crystal class is 4/mBar32/m, while the space group designation is F41dBar32/m. Twins are common on the {111} plane. It has perfect four directional cleavage, adamantine luster, and both a high refractive index, 2.42, and specific gravity, 3.52. Color is usually pale yellow to colorless, but can also be brown, blue, green, orange, red, and black. The external crystal class is 4/mBar32/m, while the space group designation is F41dBar32/m. Twins are common on the {111} plane. It has perfect four directional cleavage, adamantine luster, and both a high refractive index, 2.42, and specific gravity, 3.52. Color is usually pale yellow to colorless, but can also be brown, blue, green, orange, red, and black.