Diamond are extensively used in industry, but are better known for their use as gemstones. The value of a diamond depends on size, quality and shape. In the seventeenth century, Tavernier recorded that the value of a diamond was proportional to the square of the weight, and this is still true. The most popular cut as a gemstones is the 'brilliant', a round cut with fifty-eight facets that was developed to obtain the maximum brilliance and fire from the stone. There are also other fancy cuts like the kite, traingle and baguette. The term, 'melee' refers to stones cut from small fragments of larger rough stones, obtained after cutting. Approximately eight to sixteen such stones together weight one carat, and many of these small stones are cut with fifty-eight facets. Smaller melee stones are cut with only eighteen facets and may be as small as 0.01 carat each.
Merchants found that the dried brown fruit of the locust tree (ceratonica siliqua), that looked liked a horn (keration in Greek), had seeds of an extraordinarily uniform weight. In course of time, the seed, the keration or carat became the standard for weighing diamonds and gold. Now the standard weight for diamonds is the carat (two-hundred milligram or a fifth of a gram), rather than the more picturesque seed.
When heated in oxygen above 650 C, a coating of graphite forms on the diamond. In an inert atmosphere, the transition to graphite occurs above 1570 C. Diamond is extremely inert to acids and chemicals until heated to 1020K, but sodium nitrate attacks it at 430 C and metals react to form carbides. At pressures of 70,000 atmospheres and 2500 C, graphite may crystallise into diamond. The specific gravity of diamond is 3.5 and its refractive index is 2.4 as against 1.5 for glass and 1.33 for water. Diamond also has high dispersion and this gives 'fire' to the crystal. Diamond conducts heat but is a good electrical insulator; this is sure way to detect a real stone from fakes.