Diamond, the other crystalline form of pure carbon, is the world's hardest natural material, and is used in industry as an abrasive and in drill tips for drilling through rock in oil fields and human teeth in dentists' offices. On a hardness scale of one to ten, which mineralogists refer to as the Moh scale of hardness, diamond is awarded a perfect ten. But that's not why diamonds are so expensive. They are the most expensive of all gems, and are kept that way by supply and demand. The supply is largely controlled by the De Beers Consolidated Mines, Inc. in South Africa, where most of the world's diamonds are mined, and the demand is kept high by the importance that is widely attributed to diamonds.
A diamond can be considered to be a single huge molecule consisting of nothing but carbon atoms that are strongly bonded to each other by covalent bonds, just as in other molecules. A one-carat diamond "molecule" contains 1022 carbon atoms.
The beauty of gem-quality diamonds—industrial diamonds are small, dark, and cloudy—comes from their crystal clarity, their high refractivity (ability to bend light rays), and their high dispersion—their ability to spread light of different colors apart, which makes the diamond's rainbow "fire." Skillful chipping of the gems into facets (flat faces) at carefully calculated angles makes the most of their sparkle. Even though diamonds are hard, meaning that they cannot be scratched by other materials, they are brittle—they can be cracked.
Science EncyclopediaScience & Philosophy: Calcium Sulfate to Categorical imperativeCarbon - How Carbon Is Found, Graphite, Diamond, The Chemistry Of Carbon, Why Carbon Is Special - Classes of carbon compounds