92 GENERAL CHARACTERS OF PRECIOUS STONES 



David Brewster, who for the same purpose made use of Canada-balsam, oil of anise or 

 sassafras-oil. 



It is not difficult, by one or other of the artifices already described, to conceal or make 

 inconspicuous the various faults to which precious stones are liable. On this account, 

 therefore, it is a rule to be observed that valuable stones of high price should never be 

 purchased in a setting, but in a loose and unmounted condition, which admits of a thorough 

 and complete examination of the stone being made. Such an examination should be made 

 before the purchase not only of cut-stones, but also of uncut, rough stones. Small faults in 

 the latter are often very difficult to detect, since the roughness of the surface interferes more 

 or less considerably with the transparency of the stone. In such cases it is advisable to 

 ])lace the stone in a highly refractive liquid, which will have the apparent effect of lessening 

 the roughness of the surface and increasing the transparency of the stone. Even when 

 placed in water such stones will appear more transparent than in air. In examining a rough 

 stone it is important to determine whether any fault it may have lies quite in the interior 

 of the stone or near the surface. In the latter case it may often be removed in the process of 

 cutting, and a somewhat imperfect rough stone may be transformed into a gem of flawless 

 beauty. 



The nature of the faults which are of the most frequent occurrence in precious stones 

 may be gathered from what has already been said. Those of the nature of coloured patches, 

 enclosures of large foreign bodies and such like, need no further description ; other faults, 

 however, which occur again and again with the same characteristic appearance are 

 distinguished by jewellers with special names and receive special mention below. 



1. Sand. — Small grains of any foreign substance, of a white, brown or reddish colour 

 scattered singly through the material of the stone are known as sand. 



2. Dust. — This is the name by which extremely small particles of foreign matter 

 scattered in great numbers through the substance of a stone are known. 



3. Clouds. — By this term is meant muddy or cloudy patches of various colours — 

 white, grey, brown, reddish, greenish, &c. — which may occur in the substance of a stone and 

 which when brought to the surface in the process of cutting, give it a dull appearance which 

 no amount of polishing will remove. They are most frequent in diamonds and pale 

 rubies. 



The three kinds of faults just described are all due to the inclusion of small mineral 

 grains as impurities in the substance of the precious stone. If not too small, they may 

 sometimes be seen with the naked eye or with a good lens, but more frequently the powerful 

 magnification of a microscope is required. Their presence is best demonstrated, however, 

 when the stone is examined in polarised light ; in the dark field of the polariscope such 

 inclusions will sometimes appear bright and vividly coloured. 



4. Silk. — This term signifies the whitish, shimmering streaks, disposed in certain 

 directions, which sometimes mar the appearance of a stone. Such streaks are in reality 

 strings of microscopically small cavities in the substance of the stone, which may be quite 

 empty or may, on the other hand, contain a liquid. Such cavities are not at all uncommon 

 in precious stones ; they are sometimes of quite appreciable size, as in topaz, sapphire, &c., 

 when they may be seen with the naked eye or with a lens. The cavities, to which the fault 

 known as " silk " is due, are however definitely arranged in bands and strings, and only 

 become visible as single objects under strong magnification. By scattering at their surfaces 

 the light which should pass directly through the stone, these minute cavities produce the 

 dull, whitish shimmer. They have the same effect as " clouds" in that, when they occur on 

 the surface of a stone, a perfect polish over that area is impossible. 



