BLACK DIAMONDS.

Star (Christchurch), Issue 6901, 15 September 1900, Page 1

BLACK DIAMONDS.

(Scientific .American.) The only two regions from, whioh black diamonds are taken seem to be the Cape of Good Hope and the province of Bahia in Brazil." The black diamonds are divided into two varieties, known as carbons and borts. t The first of these is a variety of diamond which occurs in irregular crystals, having a somewhat granular structure without possessing a distinct cleavage. Its hardness is at least as great as that of the diamond, although its density is inferior on account of a slight porosity ; it has a resinous lustre and is grayish or black in colour. The "bort," on the contrary, is somewhat spherical, and does not 'present the irregularities shown by the " carbons " ; it is grayish or black, and somewhat translucent, taking the form of round masses with a rouigh surface, or one presenting a confused crystalline structure. It is only within the last ten years that the black diamond industry in Brazil has assumed any considerable importance, and the demand is increasing. The region where the blabk diamonds are found is about one day's journeyfrom Bahia, by boat to San Felix and by railroad to Bandeira do Mello; this is also a dia-mond-producing district. The most productive region is found beyond the river Paraigason, about two days' journey by mule. Ifc is probable the black diamond is fo be found in all this region, but on account of the primitive methods of extraction the only places from which they are taken are the bed of the river tm4 its tributary the San Antonio, and from the sides of the Sierra des Levras. The carbons are found in a kind of gravel consisting mainly of quartz pebbles mix«>d with ferruginous clay and resting on a clay stratum. A spot is chosen in the bed of the river where it is not more than twenty feet deep, and where the current is not too rapid; a long pole is planted, down which the native descends, being provided with a sack "whose mouth is stretched open by an iron ring. The clay is scraped from the bed of the stream and the sack filled with gravel; it is brought up and taken to the shore, the sacks being stored out of reach of high water. This operation is carried on each day for the six months of the dry season. At the comonencemtnt of the rainy season, when the search is ' suspended on account of the current and the great depth of the river, the gravel is washed and examined for carbons. The divers are quite skilful, and can remain under water for more-,than a minute and a half. The parts of the river having a great depth are not worked ; here dredlges or diving apparatus could be used to ad/vantage. Another method of extraction consists in perforating the sides of the mountain along the shore, and a number of tunnels have thus been made for the extraction of gravel containing diamonds and carbons. The work is carried on during the dry season, and in the rainy season the gravel is washed in cradles or by similar methods. The carbons are found in dimensions varying from a grain of sand to crystals weighing 975 carats ; the largest was discovered in 1894, and sold at Paris for 100,000 francs. The size preferred 's that weighing from one to three carats, as the large masses must be broken', with considerable loss. The carbons are usfd principally in the construction of rock-drills, being disposed on a steel crown in circles or rows ; their- price is naturally high's this resulting in par^ from the imperfect methods of extraction employed. The buyers are found chiefly at Bahia, and have representatives in the mining region ; the price is variable, and averages 22d0l per carat.