CATALOGUE OF GEMS. 



553 



cabochon is a characteristic cut for the turquoise. The hollow cabochon 

 lii adapted to very deep colored transparent stones. The mixed 

 cabochon has either the edge or side faceted, or both. In all of the 

 cabochon cuts the arches may be of a varying degree of flatness, 

 depending upon the nature of the stone. 



VI. IMITATIONS, SOPHISTICATIONS, AND ARTIFICIAL 

 FORMATION OF GEMS. 



Practically all of the gem stones have been and are successfully 

 imitated. Deceptions are practiced by means of imitations, pure and 

 simple; or by combining a genuine stone with an imitation, or an 

 inferior one with a superior; b)^ substituting one stone for another; 

 or by intensifying, improving, or changing the color of a stone. 



The basis of most imitation gems is a lead glass of great brilliancy 

 called paste or strass. Its composition varies considerably, but a 

 typical mixture is the following: 



Pure powdered quartz 38. 2 



Red lead 53. 3 



Potassium carbonate 7.8 



The materials are caref u'ly powdered, mixed, and heated in a crucible. 

 The temperature is gradually raised to fusion and carefully kept at 

 that point for about thirty hours, after which it is very slowly low- 

 ered. The value of the product depends entirely on the intimacy of 

 the previous admixture, the regularity of temperature, the duration 

 of fusion, and the slowness of cooling. 



Imitation diamonds are cut from the uncolored paste, but for the 

 imitation of colored gems the desired color is imparted by the solution 

 of certain metallic oxides and other substances in the paste, some of 

 the more typical mixtures being given in the following table :^ 



1 Thorpe's Dictionary of Applied Chemistry, II, p. 222. 



