282 MINERALOGY 



produce the brilliancy and luster of a well-cut, perfect stone. 

 Diamond is pure carbon, yielding on combustion carbon dioxide 

 and .05 to .20 per cent, of ash. This foreign matter is due to impuri- 

 ties or inclusions, and it is these impurities which cause the various 

 shades of color ; the yellow shades predominate, and while often 

 more brilliant are not as valuable as those of a steel- white color; 

 blue, green, or red diamonds are the most valuable of all gems. 



Cleavage fragments and dark brown specimens are termed bort 

 and are used in glass cutters, or reduced to dust for polishing. 

 Another most important use is in core drilling, a most convenient 

 and economical method of prospecting mining properties. Car- 

 bonado, a black variety, from the province of Bahia, Brazil, occur- 

 ring in rounded masses, lacks the perfect cleavage of the transpar- 

 ent stones. It is slightly porous and therefore of a lower specific 

 gravity, yet harder than the well-crystallized material, and is said 

 to yield better results in drilling than the diamond fragments. 



The commercial unit of weight used in estimating the value of 

 diamonds, as well as other precious stones, is the carat. Like all 

 of the old units of measure the carat of merchants of different coun- 

 tries varied from 188.5 to 254.6 milligrams ; the weight commonly 

 used was from 205 to 207 milligrams. The metric carat of 200 

 milligrams is now the legal carat in all countries using the metric 

 system. 



All diamonds of the ancients and of Europe until 1727, when 

 diamonds were discovered in Brazil, were from the East, where they 

 were obtained from alluvial washings and in conglomerates, espe- 

 cially at Purteal and Golconda, India. The noted diamonds ol 

 these fields are the Koh-i-noor, of 186 carats and now recut to 106 ; 

 the Pitt or Regent, a yellow stone of 137.5 carats, now in the 

 Galerie d'Apollon in the Louvre, Paris (this stone was appraised 

 in 1791 at 12 million francs); the Orloff, of 194 carats; the Blue 

 Hope, of 44.5; and probably the Great Mogul, of 279 carats. 



In 1727 diamonds were discovered by the miners in the gold 

 washings of Minas Geraes, Brazil ; since then these workings have 

 yielded continuously large quantities of good stones. Here also 

 the crystals are obtained in river washings and prairie deposits 

 and are associated with a peculiar quartz schist or flexible sand- 

 stone, termed itacolumite. The most famous diamond of the 

 Brazilian field is the " Star of the South," weighing 247.5 carats 

 uncut and 125 when cut. 



The Vaal River locality of South Africa was discovered in 1867, 





