Selling Diamonds

(via Diamond Promotion Service) 21. The diamonds are separated from the concentrate in the final steps of recovery. Until recently, this was done primarily by grease tables or grease belts. Diamonds are water-resistant; therefore when the concentrate is sluiced over tables or moving belts coated with a heavy grease, the diamonds stick to the grease and the rest of the material is washed away. Then the diamonds and grease are scraped up and put into boiling water which cleans away the grease.

22. The latest development for this final recovery is X-ray separation. A diamond fluoresces in an X-ray beam. The diamondiferous concentrate passes a beam. The fluorescence triggers a photoelectric cell, which in turn, trips a jet of air that blows the diamond out of the main stream. When another mineral passes the electronic eye, nothing happens. This method of separation is incredibly fast, and it’s more than 99 percent effective.

23. In alluvial mines, the diamonds are usually found among the gravels and other material lying on the bedrock. The sand or earthen overburden is scraped up and dumped in an other location. Then the diamond-bearing gravels are scooped up and taken away for the recovery treatment. Every crevice of the bedrock is swept clean with brooms to make sure that they get all the diamonds. The recovery process is essentially the same as for ore from the open cast and underground mines.

24. The ratio of diamonds within the diamond-bearing rock is very low. In some mines, to recover one ounce of diamonds—which is 142 carats—the miners have to dig as much as 112,000,000 ounces of material—which is 3500 tons. Furthermore, the average size of the rough diamonds is less than a carat. So in one ounce of rough—142 carats—there may be only half a dozen stones larger than a carat, and a couple of hundred smaller than half a carat.

25. Of the total world production of rough diamonds, only about 20 percent are capable of being cut into gem diamonds. The rest are put to industrial use for the cutting, grinding and polishing of a great variety of materials used in manufacturing. The gem-industrial ratio varies from country to country. Production in the Republic of Zaire (DR Congo) runs more than 98 percent industrial while that of South-West Africa is more than 90 percent gem.

26. Broadly, diamonds are sorted in the following categories: stones, which are octahedral in shape and look like two four-sided pyramids joined at their bases; cleavages, or broken and blocky pieces; macles, which are triangular stones usually with a herring-bone line around them; and flats, or thin but cuttable diamonds. Stones and shapes weighing less than one carat in the rough are known as melee. Cleavages under one carat in the rough are known as chips. Bort, ballas and carbonado are varieties of diamond used only in industry.

27. Synthetic diamonds for industrial uses are being manufactured by subjecting graphite compounds to tremendous heat and pressure in giant presses. Synthetic gem diamonds have been made in laboratory experiments, but only at great cost, far more than the cost of natural gem diamonds.

Selling Diamonds (continued)