PROCESS OF CUTTING 83 



having the chemical formula SiC, and consisting of 30 per cent, of carbon and 70 per cent, 

 of silicon. It frequently crystallises in very distinct hexagonal plates, which are translucent, 

 greenish-yellow in colour and very brilliant. It is considerably softer than diamond, but 

 scratches corundum with ease ; its great brittleness renders it easily reduced to powder in 

 spite of its great hardness. Carborundum can be obtained cheaply and with little difficulty 

 in crystalline blocks weighing a hundredweight, and it will doubtless gradually replace 

 emery for abrasive purposes. 



Finally, the hardest and most important grinding material is diamond. Diamond often 

 occurs so impure as to be useless as a gem ; this impure material is the so-called bort, which 

 when finely powdered becomes an important abi'asive agent. The black, opaque, finely 

 granular variety of diamond known as carbonado, when finely powdered also furnishes a 

 useful grinding material. In spite of the fact that even these varieties of diamond are of 

 very high price, their use in grinding precious stones greatly cheapens the process, for, 

 on account of their enormous hardness, a considerable saving in time and labour is effected. 

 Since the discovery of the South African diamond fields, the price of diamond dust has been 

 considerably lowered, and it is frequently advantageous at the present time to use this 

 material, instead of emery-powder, in the cutting of many precious stones ; the economy 

 effected in time and labour more than compensating for the higher price of the grinding 

 material. In cutting the diamond no choice of grinding material is at present possible, since 

 diamond-powder is the only substance hard enough for the purpose. 



The polishing material is varied according to the nature of the precious stone to 

 be operated upon. In the form of the very finest powder, such diverse substances as 

 tripolite, rotten-stone, jeweller's rouge, pumice-stone, putty-powder (tin oxide), and some- 

 times a variety of clay known as bole, are all employed for this purpose. The polishing 

 material, like the grinding material, is made into a paste before being applied to the lap or 

 polishing disc ; water is generally used for this purpose, but with tripolite sulphuric acid is 

 sometimes employed. 



It has already been mentioned that different precious stones are worked with different 

 grinding powders and on laps of different material, these two conditions being varied to suit 

 the particular characteristics of the stone, that of hardness receiving special attention. The 

 choice of material for the lap and of grinding and polishing powder lies between certain 

 somewhat arbitrary limits, thus the same method for one and the same kind of precious 

 stone is not always exactly followed. All precious stones may be conveniently grouped in 

 a few classes according to their hardness ; those of each group may be worked in the same 

 manner and with the same materials. The following grouping shows such an arrangement. 



a. Very hard stones. Ruby, sapphire, and other varieties of corundum. 

 Ground on an iron, brass or copper lap with diamond-powder. Emery works only very 



slowly. 



Polished on a copper disc with tripolite. 



b. Hard stones. Spinel, chrysoberyl, topaz. 



Ground on a brass or copper disc with emery. (With topaz a tin or lead disc even may 

 be used.) 



Polished on copper with putty-powder or tripolite. 



c. Stones of medium hardness. Emerald, beryl, aquamarine, zircon, tourmaline, 

 garnet, rock-ci^stal, amethyst, agate, jasper, chalcedony, carnelian, chrysoprase, &c. 



Ground on copper or tin or lead with emery. 



Polished usually on tin with tripolite or on zinc with putty-powder, sometimes also 

 on wood. 



