Diamond is pure carbon, the element that is also the foundation of life. Carbon has interesting chemical propertied that enable it to form a truly vast number of compounds with many other elements. Some of these are biologically active. Another form of pure carbon is the mineral pencils (mixed with clay for this application), and so greasy that it is widely used as a lubricant. Yet diamond is the hardest known substance, ad will easily scratch any other material.
Diamond crystals occur in a variety of shapes and forms. The most common shape resembles two four-sided pyramids arranged base-to-base and is known as an octahedron, an eight-sided form. The direction forms seen on diamond are parallel to the octahedral faces. Other forms seen on diamond crystals include the cube and dodecahedron, the latter an interesting 12-sided form. Some rough diamond crystals are combinations of several of these forms. Also frequently present on diamond crystal surfaces are triangular pits called trillions, believed to have formed during crystal growth.
Diamonds occur in a wide range of colors. The most familiar are basically white or colorless, usually with a tinge of yellow or gray. Richly colored stones, called fancies, are rare and highly prized. Fancy colors include golden- yellow, blue, green, pink, and amber.
Fine yellow diamonds with so-called “canary” color make notable gems. Two of the best-known yellow diamonds are the Florentine (137.5 Carats) and the Tiffany (128.5 carats).
Brown and coffee-colored diamonds are not as rare, but are seldom seen in the trade. Orange diamonds are popular in South Africa, but few are sold in the United States. Green diamonds are very rare, the most famous being the Dresden green of 48.5 carats. Diamonds are occasionally found in a greenish-yellow hue, sometimes called “champagne color.” Blue diamonds are extremely rare. By far the most famous is the Hope Diamond, weighing 44.5 carats, on display at the Smithsonian institution in Washington, D.C. This remarkable gem has a somewhat metallic surface luster that reflects light and makes the gem appear darker than it actually is. Pale-blue diamonds are also known, and violet gems are occasionally found. Pale-red and pink diamonds are very rare and although red diamonds have been reported, an intense, fine, red gem gas not yet been authenticated. Brownish-red diamonds, on the other hand, are occasionally seen in the diamond trade.