DIAMONDS ARE FOREVERDiamond PropertiesThe fact of being the hardest substance known by men has definitely something to do with the statement: "Diamonds are forever."Diamonds are given a value of 10 in the Mohs hardness scale. Created by the German mineralogist Friedrich Mohs, the scale indicates relative hardness of substances on a rating from 1 to 10. 
Its hardness, exhibited in its resistance to scratching, is not a constant quantity, but varies in every diamond with the crystallographic direction, being greater on surfaces parallel to the octahedron than on those parallel to the dodecahedron planes.Moreover, hardness on the same face or surface varies with the direction of the cut. Diamond chrystalizes in the isometric system. Octahedra and rhombic dodecahedra are the crystal forms most commonly found, but cubes and other forms also occur. Rounded, distorted, and twinned crystals are also common. Crystalline diamonds always cleave cleanly along planes parallel to the faces of an octahedron. The specific gravity of diamonds ranges between 3.15 and 3.53, but the value for pure crystals is almost always 3.52.
Colorless stones, known as white diamonds, are extremely valuable, while yellow or brown tinged stones are regarded as imperfect. Green and blue diamonds are rarities and red diamonds are the rarest of all. Color in diamonds is caused by the presence of minor elements.
Two important characteristics of the diamond when used as a gem are its brilliancy and fire. Both the index of refraction and the dispersion (the physical properties that determine luster and fire) are higher for diamond than for any other natural, transparent, colorless stone.
Uncut diamonds have a greasy luster and are not brilliant, but when cut the same stones exhibit a high luster, characterized technically as adamantine. The effect of the high dispersion is to separate the colored components of white light so that the stone sparkles when properly cut. Some diamonds exhibit fluorescence when exposed to sunlight or other ultraviolet-light sources. The color usually is light blue, but yellow, orange, milky white, and red fluorescence may occur in some stones.
Most diamonds are not good electrical conductors and become charged with positive electricity when rubbed. Another important physical characteristic of the diamond is its resistance to attack by acids or bases. Transparent diamond crystals heated in oxygen burn at about 800° C (about 1470° F), forming carbon dioxide.
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Diamonds exhibit a wide range of transparency and color. Good quality diamonds of clear, strong, and unusual color are highly prized.
Other characteristics of the diamond add nothing to its appearance but are frequently useful in identifying the stone and in differentiating between true diamonds and imitations. Because diamonds are excellent conductors of heat, they are cold to the touch.