APPLICATION AS JEWELS 71 



emery. The use of these materials in the cutting, grinding, and polishing of precious stones 

 will be dealt with below in the special description of these processes. 



The diamond, on account of its extreme hardness, has many other technical applications, 

 which will be noticed in detail further on under the special description of this stone. It is 

 used for engraving and boring precious stones and other hard materials, while its use in 

 rock-drills for mining and other operations is scarcely less extensive than its use as the 

 glazier's diamond. 



B. APPLICATION AS JEWELS. 



The use of precious stones as gems is much more extensive and varied than for any 

 other purpose. In their rough state they have not, as a rale, a pleasing appearance, and 

 therefore are unsuitable for this purpose ; it is only after cutting and polishing that their 

 beauty appears in all its fulness. 



The process of cutting aims at giving each stone such a form as will best display its 

 natural lustre and beauty. Thus the form in one case may be rounded, in another bounded 

 by small faces or facets, the latter being very frequently used. The various modes of 

 cutting in vogue at the present day, each of which is best suited to the idiosyncrasy of the 

 particular stone to which it is applied, are the results of centuries of trial and observation 

 on the part of gem-cutters. Thus the form in which transparent stones are cut differs from 

 that best suited to opaque stones ; and in the same way, the form in which dark-coloured 

 stones are cut differs from that given to lighter or colourless specimens. The appearance 

 of each would suffer if it were given any other than its own appropriate form. 



The amount of refraction and dispersion exercised upon light by a transparent stone 

 greatly affects its appearance, as has been shown in the case of diamond. It has also been 

 shown that to obtain a maximum effect, the greater part of the light which enters by the 

 front facets of a cut stone must be reflected from the back facets, and must again pass out 

 by the front facets. Since the path of a ray of light in a stone varies with the refractive 

 index of the stone, and this character is different in different stones, it follows that the 

 form of cutting must be adapted to the requirements of each particular case. It is thus the 

 task of the gem-cutter to give to each stone that form which is calculated to bring out and 

 display its beauties to the greatest possible advantage, and which, at the same time, 

 involves the least possible waste of valuable material. 



Gem-cutters, by prolonged experience, have arrived at certain empirical rules which 

 are always applied, and which are modified to suit particular cases. In colourless stones, 

 for example, there must be a fixed proportion between their breadth and their thickness, 

 and there should be also certain relations between the shape of the back and that of the 

 front of the stone. 



Too great depth in a cut stone is as inimical to the full effect of its beauty as is too 

 great shallowness. In the one case a stone is said to be thick or " lumpy," and in the other 

 thin or " spread."" Of two similar stones, one of which errs on the side of too great depth, 

 and the other on that of too great shallowness, the latter is to be preferred. The facets at 

 the back of the stone must occupy a certain position relative to the front facets, otherwise 

 the light entering by these will not be totally reflected from the back. 



The same rules apply also to coloured stones. In this case the depth the stones are 

 cut is important, and this must vary with their intensity of colour. A deeply coloured 

 stone if too thick will appear dark or almost black, while a pale coloured stone will not 

 exhibit a sufficient depth of colour unless it is cut of some thickness. 



