ARTIFICIAL. PRECIOUS STONES HEATON. 227 



With this apparatus a boulc weighing some 20 to 30 carats, and 

 capable of yielding two cut stones of about 6 carats each, can be 

 prepared in about half an hour almost automatically, a single operator 

 being able to control several machines. The boules, on cooling, very 

 often split in half in the direction of their growth, as in the lower 

 example seen in figure 2, and this is a convenience rather than other- 

 wise, as the resulting shape can be cut to greater advantage. 



In the first instance reconstructed rubies were made in this way 

 after the manner introduced by Gaudin, the material fed into the 

 blowpipe being pulverized rubies and chips, and this method is still 

 employed by some workers. But more commonly nowadays the 

 corundum is produced direct from amorphous alumina by using 

 pure ammonium alum as the raw material. On reaching the flame this 

 decomposes, the ammonia and sulphuric acid volatilizing, leaving the 

 alumina. Stones made by this process are generally known as J 'syn- 

 thetic," as distinct from "reconstructed," although, of course, to be 

 pedantic, the process is one of decomposition rather than synthesis. 



The "synthetic" corundum produced in this way, if pure ammo- 

 nium alum is used, is of course colorless, and can be used as artificial 

 white sapphire. If a small proportion of chrome alum is added, the 

 resulting stones are rubies, and other colors may be produced in the 

 same Avay. For a long time all attempts to reproduce the fine blue 

 of the sapphire failed, because following the apparent analogy of 

 silicates, cobalt was invariably employed as the coloring agent. This, 

 however, does not readily form an aluminate in the same way that it 

 does a silicate, and, in consequence, it is impossible to produce a satis- 

 factory coloration in the corundum by its means ; it is possible to get 

 the cobalt in a state of combination by adding a large proportion of 

 magnesia to the alumina, but then the product formed is not a crystalline 

 alumina but magnesium aluminate, and its properties are fundamen- 

 tally different. Its refractive index is lower, its refraction single, and its 

 hardness lower. In fact, the result is blue spinel instead of sapphire. 

 Moreover, such blue stones have the characteristic absorption of 

 cobalt, and appear purple in a light that does not contain a large 

 proportion of blue rays. 



In 1908 Paris attempted to avoid this latter difficulty by preparing 

 a calcium aluminate colored with cobalt, as it is found that in this 

 case the transmission of the red rays is less pronounced. But the 

 calcium aluminate so formed is not crystalline at all, but amorphous. 

 A year or so ago, however, the problem of producing synthetic sap- 

 phire was finally solved by the use of titanium oxide, a very unex- 

 pected result, considering the chemical position of this element. With 

 this last advance the artificial production of the corundum gem stone 

 may be considered to be completely solved, and cut stones can now 

 be obtained in every variety of color, from pure white to ruby and sap- 



