Fewer than 20 percent of the diamonds mined each year are suitable for use as gems. Most are sold at monthly "sights" through the Diamond Trading Company in London. At a "sight" the buyer is presented with a parcel of uncut stones to examine; the buyer must either purchase or reject the entire package without choosing among the various stones. The stones are finished in various kinds of cuts, or "make." The best proportioned ones throw back the most light. No universal standards have been adopted for " make, " but the American Standard Brilliant Cut is the closest to ideal proportions. Finished stones are graded according to quality and then marketed. Various classification systems --based on color, clarity (freedom from flaws and inclusions), cut, and carat weight--are used to determine quality and, thereby, the market value of gem diamonds.
The largest single rough diamond ever found, the Cullinan diamond, found in 1905 in South Africa, weighed 3,106 carats. Several other large stones have been found, including the Excelsior (1893), weighing 995.2 carats and also from South Africa, and the Star of Sierra Leone (1972), weighing 969.8 carats and from Sierra Leone. Several fine gems were cut from the Cullinan, including the world's largest, the Star of Africa (530.2 carats), now in the royal scepter of the British crown jewels. The Hope diamond, an infamous deep - blue diamond of 44.5 carats, most likely came originally from India. It is now in the gem collection of the Smithsonian Institution in Washington, D.C. It demonstrates the beauty of "fancy," or highly colored diamonds, which are also found in shades of yellow, pink, champagne, and other colors. Industry uses most uncut diamonds.
Diamond-studded rotary bits are used to drill oil wells and bore tunnels in solid rock. Much low - grade diamond is crushed to dust, sorted by grain size through special sieves, and used as abrasive powder. Depending on the kind of abrasion or grinding needed, the powder is either sintered into metal disks, formed in carbide grinding wheels, pressed into metal, or mixed in an oil paste. The powder is also used to cut and polish gems. Diamond -tipped glass cutters, glass etching pencils, and other similar tools find widespread use. Very thin wire is formed by pulling thick wire through a graduated series of diamonds with tiny holes drilled through them. Diamonds for industrial purposes have been synthesized since the 1950 s using high temperature, high-pressure techniques, and since the 1960 s using shock-wave techniques. ( Gem- quality diamonds can also be synthesized , but the process is costly.) Technological uses for diamonds were expanded the late 1980s by the development of methods for depositing diamond coatings on surfaces. Such uses include the coating of integrated circuits as a whole instead of having to cool the components of the circuits individually. The coatings may also be used in prosthetic devices and biosensors. Can be irradiated to change or improve color.