CHAMBERS'S INFORMATION FOR THE PEOPLE. 



are diamond-powder and oil. In this way, a great ' 

 number of these facets, perfectly regular in shape, 

 are, solely by the eye, produced, and by con- 

 tinuing the operation they are polished. Dia- 

 monds are cut into three forms, termed the 

 brilliant, the rose, and the table, which will be 

 understood by a glance at the following figures : 



Besides its use as a gem, the diamond has been 

 long employed by glaziers for cutting glass. 

 Those which are black in colour or full of flaws 

 are termed bort or boart, and are crushed to powder 

 for the purpose of cutting the finer diamonds and 

 other gems. Bort is, however, a costly material ; 

 but there is a peculiar, dark, opaque variety of 

 the diamond, recently found in South America, 

 called carbonado, which is now used for the same 

 purposes, and is obtained at a considerably lower 

 price. Carbonado is duller in appearance than 

 the true diamond, but is of the same hardness. 

 It is also employed in the dressing of millstones, 

 and for boring hard rocks in the process of 

 tunnelling. 



Sapphire Ruby. Several gems which consist 

 solely of crystallised alumina or corundum, and 

 merely differ in colour, are in a scientific sense all 

 the same mineral, although different names are 

 given to them by lapidaries. If the colour of this 

 mineral is blue, it is called a sapphire ; if red, a 

 ruby ; if yellow, an oriental topaz ; if purple, an 

 oriental amethyst ; and so on. The sapphire is 

 next to the diamond in hardness, and next to it 

 and the emerald in value. Both the sapphire 

 and ruby are found chiefly in Ceylon and Pegu. 

 A fine ruby is the most costly of all gems. 



When the substance of which the sapphire and 

 ruby are composed occurs opaque, dingy in colour, 

 and crystalline, it is called corundum; and when 

 impure and granular, it is termed emery. Corun- 

 dum is found in India, Ava, and China, and is 

 extensively used for cutting gems and polishing 

 steel. Emery is still more largely employed for 

 grinding and polishing metals, and especially 

 plate-glass. It is used both in powder and glued 

 to paper, and is chiefly brought from the island 

 of Naxos in Greece. 



Emerald Topaz Spinel Ruby Garnet, &*c. 

 The gems in this group are compounds of silica 

 with various bases. The emerald is a silicate of 

 aluminium and glucinum the latter a rare earth, 

 and the only rare substance found in any of the 

 finer gems. Its colouring matter, hitherto sup- 

 posed to be oxide of chromium, has been recently 

 asserted to be really an organic substance analogous 

 to the green colouring matter of leaves. The 

 most celebrated modern locality for emeralds is 

 the mine of Muzo in New Granada. The beryl 

 is a pale-coloured emerald, called, when trans- 

 parent, aquamarine. What is properly called the 

 topaz is a silicate of aluminium with fluoride of 

 silicon or aluminium. The finest kinds come 

 from Brazil, and many of these are of a fine, deep, 



400 



clear yellow colour. The spinel is a compound 

 of aluminium, magnesium, and oxygen, and of 

 the same hardness as the topaz. When of a 

 scarlet colour, it is called spinel ruby; when rose- 

 coloured, balas ruby ; when orange-red, rubicelle; 

 and when violet, almandine ruby. The garnet 

 is a silicate of aluminium and calcium, or instead 

 of calcium there may be magnesium, manganese,, 

 iron, or chromium. It is about the same hard- 

 ness as rock-crystal, and occurs of various colours, 

 but it is only those which are transparent, and of 

 some shade of red, which are used in jewellery. 



The turquoise is an opaque stone of a fine 

 azure-blue colour, and much used in jewellery. It 

 consists chiefly of phosphate of aluminium, has 

 a waxy lustre, and is but moderately hard. It 

 is found in India and Tibet. 



Amethyst, Cairngorm, Cornelian, Onyx, 6r*c. 

 The ornamental stones obtained from quartz and 

 its varieties are so numerous, that we can here 

 only notice some of the principal kinds used by 

 the jeweller. Rock-crystal, already referred to, is 

 the type of the crystalline and vitreous varieties. 

 Those kinds, again, which are amorphous, trans- 

 lucent, and of waxy lustre, are called chalcedonic,. 

 and include carnelian, sard, agate, and onyx. 

 A third group comprises the jaspery varieties, 

 such as jasper and blood-stone. These substances 

 are all composed of silica more or less pure, and 

 usually coloured by veiy small quantities of other 

 bodies. The amethyst is a purple or violet 

 coloured rock-crystal. Cairngorm stone, whose 

 colour ranges from a dingy brown to a pure 

 yellow, is also a variety of rock-crystal. Both 

 are chiefly obtained from Brazil Carnelian is a 

 variety of chalcedony, usually of a lively red, but 

 sometimes of a yellow tint. Sard only differs 

 from it in being of a rich brown colour, which 

 appears blood-red by transmitted light. But it 

 is difficult to draw a line of distinction between 

 them. The finest carnelians are brought from 

 Arabia, and from Cambay and Surat in Indiiu 

 The onyx and sardonyx are also varieties of 

 chalcedony, in which the mineral occurs in bands 

 of different colours. Jasper is a variety of quartz 

 distinguished from those already mentioned by its 

 being opaque. It is often richly coloured and 

 mottled ; and more rarely is beautifully striped, 

 in which case it is called ribbon-jasper. Blood- 

 stone is a dark-green kind of jasper, thickly dotted 

 over in the finer examples with red spots, hence 

 its name. 



Opal. The noble or precious opal is also a kind 

 of quartz with the addition of from five to ten 

 per cent, of water. Many persons consider a fine 

 opal, with its wonderful play of prismatic colours, 

 the most lovely of all gems. Opal is the softest 

 of the stones which are composed of silica, and 

 it is also brittle, so that it requires to be polished 

 with great care. It is, besides, apt to lose its 

 beauty if exposed to any considerable heat, and 

 has the curious property of being always most 

 brilliant on warm days. The localities for the 

 precious opal are few, Hungary being the country 

 where the finest are obtained. Stones as large 

 as an inch in diameter are very rare, and if of 

 great beauty, will fetch one thousand pounds and 

 upwards. There is one in the Imperial Museum 

 at Vienna whose size and splendour are unique. 

 It weighs seventeen ounces, and is valued at fron>- 

 fifty to seventy thousand pounds. 



