ABRASIVES 



29 



DIAMOND 



Diamond, carbon crystallized in the cubic system, 

 is the hardest natural substance known. Of all the 

 diamond produced, only a small fraction is suitable 

 for gemstones owing to inclusions, polycrystallinity, 

 twinning, unpleasing color, or other flaws. The ma- 

 terial unsuitable for gemstones is used in industry. 

 Only a small portion of industrial diamond goes 

 into manufactured goods such as phonograph 

 needles, instrument bearings, and wire-drawing 

 dies; most is used for abrasives. Pencillike tools 

 for scribing or cutting glass, or for truing and 

 dressing abrasive wheels, have a whole diamond 

 crystal or a large fragment set into the face of the 

 tool. Rock drills also have their working faces im- 

 planted with diamond crystals, commonly in an 

 annular pattern at the end of a steel pipe. By rotat- 

 ing the pipe, a cylindrical core of rock can be cut 

 from the earth for inspection. Most industrial dia- 

 monds are crushed to powder and used in a slurry 

 of oil or water for faceting gems — especially dia- 

 mond, which can be abraded by virtually no other 

 material, drilling holes in instrument bearings, or 

 polishing optical parts; some are bonded to wheels 

 or disks and used for such purposes as slabbing rock, 

 sharpening carbide and other hardfaced tools, and 

 precision-grinding hard ceramic forms to tolerances 

 of a few millionths of an inch. 



Diamond abrasive is of critical importance to 

 industry. Until 1955, the entire supply was im- 

 ported because the United States has no commer- 

 cial deposits of its own. In 1955, a commercial 

 process for producing artificial industrial diamonds 

 was perfected in the United States. Production in 

 the United States now is in excess of 15 million 

 carats annually, and it continues to rise. It is sup- 

 plemented by at least three other manufacturers 

 throughout the world. The drop in imports of 

 natural industrial diamond by the United States has 

 not yet matched the rise in production of artificial 

 diamonds because of increasing industrial activity 

 and the discovery of new applications for abrasive 

 diamond. Imports have dropped, however, from a 

 high of about 20 million carats to about 13 million 

 carats (about 3 tons) annually and are expected to 

 drop further as economics and the increasing capa- 

 bilities of the existing and anticipated manufac- 

 turers of artificial diamond allow. 



Most imported diamond comes from the Union of 

 South Africa, but because a single distributing cor- 

 poration markets nearly 90 percent of the world 

 diamond production, the purchase of large lots of 

 industrial diamond probably implies the inclusion 

 of small quantities from such other diamond- 



producing countries as Angola, Congo, Rhodesia, 

 Ghana, Guinea, Ivory Coast, Liberia, Sierra Leone, 

 South-West Africa, Tanzania, Brazil, Guyana, In- 

 dia, Indonesia, and Venezuela. Diamond is known in 

 the Soviet Union in the Ural Mountains and at the 

 recently discovered deposits near Yakutiya; but 

 little, if any, Soviet diamond comes to the world 

 market. 



Natural diamond normally occurs only in an un- 

 usual type of peridotitic igneous rock known as 

 kimberlite which was injected into overlying rocks 

 mostly as cylindrical bodies, or "pipes," from deep 

 in the crust of the earth. Of several hundred pipes 

 known throughout the world, only a few contain 

 diamonds, and of these only a few contain diamonds 

 in commercial quantity. Two kimberlite pipes are 

 known near Murphreesboro, Ark. One contains un- 

 economic quantities of diamond; the other, appar- 

 ently, no diamond. Although diamond has been 

 found in at least half the States, neither the original 

 diamond-bearing pipes nor the alluvial placer con- 

 centrations derived from them have been discov- 

 ered. The few stones found in the North-Central 

 States probably were carried south from undiscov- 

 ered Canadian sources by the continental glaciers 

 of the fairly recent past. 



It is at least possible that diamonds can form in 

 highly metamorphosed rock; these are common in 

 the Southeastern States, California, Montana, and 

 elsewhere where diamonds have been found. But 

 ultramafic rocks also are known in these areas, and 

 kimberlite pipes cannot be ruled out as the sources 

 of these diamonds simply because they have not yet 

 been discovered. Usually neither the topographic 

 expression of pipes nor the surface appearance of 

 weathered kimberlitic material is so exceptional as 

 to indicate readily the presence of pipes, and the 

 pipes are easily hidden by surficial material because 

 they do not cover large areas — the largest diamond 

 pipe known in the world covers barely 350 acres. 

 Perhaps the best way to prospect for kimberlites or 

 other diamondiferous rocks is to pan stream samples 

 for diamonds, which are heavy and remain in the 

 concentrate, and especially to look for those other 

 few heavy and unusual minerals that almost uni- 

 versally accompany diamond — chrome diopside and 

 pyrope garnet, together with the more common 

 ilmenite. 



CORUNDUM 



Corundum is aluminum oxide crystallized in the 

 hexagonal system. After diamond it is the hardest 

 known natural substance. Two forms of corundum 

 are common : ( 1 ) prismatic crystals which range in 

 size from sugary aggregates to large crystals that 



