USEFUL MINERALS. 



PRECIOUS STONES. 



We have already given an account of some 

 minerals which are semi-precious, such as jet, 

 amber, coral, and jade. A few others of this 

 class, as the amethyst, the turquoise, the garnet, 

 and the onyx, will be briefly referred to in this 

 -section, which, however, will be mainly devoted 

 to the more valuable precious stones or gems. 

 These are the diamond, the ruby, the sapphire, 

 the emerald, the spine!, and the noble opaL The 

 pearl, although a gem, and largely composed of 

 -carbonate calcium, belongs properly to ZOOLOGY, 

 as a product of the animal kingdom. All 

 precious stones occur in small crystals, often 

 -worn into a rounded form ; or in amorphous 

 masses and concretions in some rocky matrix. 

 When found in the sand or gravel of water- 

 courses, they have been washed out of the parent 

 rock. So rare and so limited in size are the finer 

 rgems, that when they approach the size of a 

 pigeon's egg, they become of enormous, some of 

 -them of fabulous value. Most gems are com- 

 posed of the same substances as many of our 

 commonest minerals. 



Diamond. This well-known gem surpasses all 

 others in hardness and brilliancy. No other sub- 

 -stance will scratch the diamond, and it is not 

 acted upon by acids and alkalies. It is, however, 

 as first suspected by Newton, combustible, and 

 when heated in a vessel containing oxygen, it 

 "burns with facility, yielding carbonic acid gas, 

 thus proving that it consists of pure carbon. Its 

 power of refracting light is very high, a property 

 which confers upon it much of its splendour. It 

 is believed that the true matrix of the diamond 

 has not yet been discovered, although it is some- 

 times found in a loosely adhering conglomerate 

 in Brazil, and in a friable calcareous rock at the 

 Cape. If it be, as generally believed, of vegetable 

 origin, it may never have had a rocky matrix like 

 other gems. At all events, diamonds are usually 

 washed out of loose soil of various kinds, but 

 always, according to Dana, out of the soils of gold- 

 bearing regions. They occur crystallised in the 

 form of octahedrons, dodecahedrons, and other 

 modifications of the cube, but with the sides and 

 angles often rounded. 



Until the early part of last century, all the 

 diamonds of commerce came from India, where 

 they are still found, but very sparingly. The 

 famous mines of Golconda, which are said to have 

 been at one time let for ,150,000 per annum, 

 with the reservation of all diamonds above ten 

 carats in weight, and to have employed thirty 

 thousand labourers, are now let to some natives 

 for less than twenty shillings a year, who consider 

 themselves lucky if they find a stone worth eight 

 or ten shillings in a month. Borneo is perhaps 

 the only Eastern country from which diamonds 

 are still brought, and the number imported is 

 comparatively smalL For nearly one hundred 

 and fifty years the world has been chiefly supplied 

 .from the Brazilian mines, the most celebrated of 



which are on the rivers Jequitinhonha and Pardo, 

 north of Rio Janeiro, where the sands are washed 

 by slaves. From first to last, about two tons of 

 this gem have been obtained in Brazil In the 

 present century, diamonds have at different times 

 been procured in the Ural Mountains ; and since 

 1860, some have been found in New South Wales, 

 but none of much commercial value. A new 

 diamond-field of rich promise was, however, dis- 

 covered in South Africa in March 1867. The 

 locality is on the Vaal River, about eight hundred 

 miles from Cape Town, and the diamond territory 

 there is now known to stretch over one hundred 

 miles. Kimberley is the headquarters of dia- 

 mond mining, and by 1881, stones to the value of 

 ^23,000,000 had been found. The official returns 

 give the value raised in 1883 at 2,742,4.70. The 

 Porter Rhodes diamond found there weighed 150 

 carats uncut, and ^60,000 was refused for it 



Precious diamonds are most usually transparent 

 and colourless, but they are found of various 

 colours, as red, yellow, green, blue, brown, and 

 black. It is commonly stated that perfectly pure 

 and colourless diamonds, technically said to be 

 ' of the first water,' are of the highest value, but 

 this is really not the case. High authorities say 

 that those with a decided colour, such as blue, 

 red, or green, bring by far the most exorbitant 

 prices. Thus a diamond of a bright green colour 

 weighing five grains has sold for ^320, which> 

 if it had been white, would not have fetched more 

 than ,28. Just before the discovery of the Cape 

 field, a fine brilliant weighing half a carat was 

 valued at $, los. ; one of one carat, at .18 ; one 

 of two carats, at ^65 ; one of three carats, at ^125 ; 

 one of four carats, at ,220 ; and one of five carats, 

 at .320. Six years after the discovery of Cape 

 diamonds, yellows under five carats were quoted 

 at from 405. to 505. ; above that weight, from 3 

 to 4 per carat ; pure white stones under five 

 carats, 3 to 4 above five carats, 4 to 7. A 

 carat is equal to 3^ troy grains. 



The largest authentic diamond known is the 

 ' Mogul,' found at the Gani mine, seven days distant 

 from Golconda, and weighs 280 carats. Other 

 famous diamonds are the Orloff or Russian 

 diamond, weighing 193 carats ; the Koh-i-noor, 

 weighing, since it has been recut, 102^ carats ; 

 the ' Regent of France,' said to be the most per- 

 fect in existence, weighing 136! carats; and the 

 Hope diamond, of a unique blue colour, weighing 

 44^ carats. There are still two stones much 

 larger than any of these, but the fact of their 

 being real diamonds is doubted. 



The processes in diamond-cutting as at present 

 practised were shewn by M. Coster of Amster- 

 dam in the Paris Exhibition of 1867, and are as 

 follows : The diamond is first chipped into shape 

 by a hammer and steel knife, an operation re- 

 quiring great dexterity, because it will only split 

 in the direction of the cleavage planes ; and 

 neither too large nor too small a piece must be 

 struck off. Two diamonds, each fastened to a 

 stick with cement, are then rubbed together till 

 the principal faces are, one after another, formed 

 two facets, that is, one on each stone, being of 

 course made at the same time. When the chief 

 facets are thus shaped out, the diamond, in order 

 to have their number increased, is attached to a 

 tool by means of a fusible metal, and held down 

 on a rapidly revolving iron disc upon which there 



