98 GENERAL CHARACTERS OF PRECIOUS STONES 



silvery line, which is not the case when the material is a true precious stone. Glass, being an 

 amorphous substance, is singly refracting, while many precious stones are doubly refracting ; 

 this difference may, with the help of the polariscope, prove useful in distinguishing the two. 

 Again, singly refracting glass is never dichroic, hence a stone which, when examined with the 

 dichroscope shows different colours, cannot be glass. Some precious stones, for example 

 diamond, are, however, singly refracting, and not dichroic : these could not be distinguished 

 from ordinary glass by the aid of the polariscope alone, unless, indeed, the black cross 

 shown by unannealed glass, as already mentioned, should happen to be observed. There is 

 usually a difference in the specific gravity of a stone and of its glass imitation ; with very 

 heavy glasses, however, the specific gravity of which may be as high as 3 6 to 3"8, this 

 character may in certain cases approach more or less closely that of the pi'ecicus stone it 

 imitates. Finally, it is to be noted that the manufacture of a glass absolutely free from 

 small air bubbles and other irregularities, such as streakiness, banding, &c., which never 

 occur in precious sbones, is very difficult. Observation with a lens, or when necessary with a 

 microscope, will often result in the detection of such bubbles and streaks, the presence of 

 which will prove the false character of the stone. Moreover, there may often be seen at 

 the edge or girdle of a faceted specimen the marked, conchoidal fracture of glass, which 

 is often present in a characteristic manner, different from anything seen in a genuine 

 precious stone. 



The material used for the production of imitations of precious stones is, in most cases, 

 a readily fusible, colourless glass, rich in lead, and known by the names strass, paste, or 

 Mainz flux. The qualities which this substance must show before everything else are the 

 most perfect transparency and clearness and freedom from colour ; it is therefore of 

 importance that the raw materials used in its manufacture should be of the greatest possible 

 purity. The constituents of strass are, as a rule, the same as those of ordinary glass with 

 the addition of one or two other substances, especially of red-lead. The most important 

 constituent is quartz, which must be quite free from iron, and is best suited for this particular 

 use in the purest form of I'ock-crystal. Potash is used in the form of potassium carbonate 

 (potashes), which must also be as chemically pure as possible ; potassium nitrate often 

 replaces the carbonate, since this salt can be more easily obtained in a pure condition ; for the 

 same reason another salt, potassium tartrate, is sometimes used. Potash, as a constituent 

 of strass, is sometimes replaced altogether by thallium, which can be used in the form 

 of any of its salts, the product thus obtained being known as thallium-glass. Lead is 

 employed in the form of red oxide (red-lead), which is prepared from chemically pure 

 metallic lead. A little white arsenic is sometimes added, but this is not an essential 

 constituent, and because of its poisonous nature is often omitted. For the purpose of 

 increasing the fusibility of the mixture, a little borax or pure boracic acid is added as a flux ; 

 this, however, does not enter into the composition of the glass, but is volatilised by the heat 

 of the glass-furnace. 



These materials, after being powdered finely and intimately mixed, are fused together in 

 a Hessian crucible, and kept at as constant a temperature as possible, which should not be 

 higher than that just sufficient to produce complete fusion. The fused mass, which should 

 then be homogeneous, and as free from bubbles as possible, is allowed to remain for about 

 twenty-four hours in the furnace, during which time it cools gradually and slowly. Any 

 disturbance of the fused mass must be avoided in order to guard against the introduction of 

 air bubbles, which cannot be again expelled, and which render the product unsuitable for the 

 purpose for which it is intended. 



The constituents mentioned above are not used in the same proportions in all cases. 



