GENERAL METHODS 565 



discovered by which glass imitations can be distinguished from genuine precious stones. 

 An aluminium pencil drawn across a clean, dry surface of glass leaves a metallic silvery 

 streak, but on the surface of a precious stone the aluminium leaves no mark. 



The behaviour of a precious stone towards acids is another character of which only a 

 limited use can be made. It is useful, for example, when carbonates, such as malachite, are 

 in question, for a drop of acid placed on a rough stone, or on the back of a polished one, 

 causes a brisk effervescence, and does no particular damage. The fusibility can only be 

 determined upon fragments of rough stones, and, for our present purpose, is of little 

 importance. 



The magnetic and electrical characters of a stone are often useful in determining 

 its identity. The phenomenon of pyroelectricity exhibited by tourmaline to such a 

 marked degree often serves to identify this stone, and the existence of this property can be 

 demonstrated without the slightest risk of injury to the stone. The supposed tourmaline is 

 warmed in an air-bath or on a sheet of paper held over a flame ; and while cooling it is 

 dusted over with a mixture of red-lead and sulphur shaken through a sieve. Mutual 

 friction causes the red-lead to become positively, and the sulphur negatively electrified, and 

 if the crystal during cooling has acquired statical charges of pyroelectricity, the red-lead 

 will be attracted to the negative pole of the crystal and the sulphur to the positive 

 pole. 



Quite recently it has been discovered that the Rontgen (X) rays afford another 

 means whereby certain precious stones may be distinguished from their imitations. The 

 most important case is that of the diamond. This is perfectly transparent to the rays, but 

 •glass, topaz, rock-crystal, &c., are opaque, and will appear in a photograph taken by X rays 

 with sharp outlines. Corundum (ruby and sapphire, &c.) is semi-transparent, while spinel 

 and blue tourmaline, like glass, are opaque, so that here again the former may be 

 distinguished from the latter. Amber and other similar resins, as well as jet, are transparent 

 to a marked degree, and can, therefore, be distinguished from glass imitations, which are 

 opaque to the rays. The whole method is as yet in its infancy, but will no doubt become 

 important as a ready means of establishing the identity of a diamond, whether mounted or 

 unmounted, without injury to the stone. Moreover, a large pai'cel of diamonds may 

 be photographed, and the presence of any stone not diamond readily detected. 



In what follows, the characters described above will be utilised to distinguish precious 

 stones of similar appearance one from another. The first step is to classify all stones into 



three main groups : 



(a) Transparent stones. 



(6) Translucent and opaque stones. 



(c) Stones with special optical effects. 



A classification based upon transparency or translucency is difficult to follo^v in 

 practice, for large specimens of ordinarily transparent stones are sometimes only translucent, 

 and one specimen of the same mineral species may be perfectly transparent while another 

 is perfectly opaque. In the scheme which follows, therefore, stones which may be either 

 transparent or translucent will be placed in both classes. The stones included in classes 

 a and b are arranged in sub-groups according to colour, and those in the third class 

 according to the nature of the optical phenomenon to which their characteristic appearance 

 is due. 



