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The hardest mineral on the Mohs hardness scale; a stone extremely resistant to scratching (although rather brittle and prone to cleavage along certain planes of the crystal structure). About 80 percent of all diamonds mined are used for industrial purposes, rather than jewelry.

Still, diamonds are the single most popular gemstone in the world, usually transparent but occasionally naturally colored diamonds such as the Hope Diamond are found. These are usually more highly prized and expensive than the transparent crystals. Some colored diamonds are enhanced, that is, they were originally transparent but have been heated until they change color; these are still valuable but no more so than a transparent diamond. In general, the price of a diamond is determined by its quality according to the four Cs of diamond grading.

Southern Africa currently produces the majority of gem-grade diamonds, with some also coming from Australia. Laboratory-made synthetic diamonds are available but not yet very popular; simulants such as cubic zirconia are often seen but are easy for a gem expert to tell from a real diamond.