ARTIFICIAL. PRECIOUS STONES HEATON. 229 



may be present in the liquid are distorted from their natural shape so 

 as to accord with this symmetrical growth. It is the great exception 

 to find a natural ruby entirely free from such inclusions, which 

 generally form irregular cavities with a decided tendency to geometri- 

 cal shape. 



It is very common also to find the structure technically known as 

 "silk" caused by microscopic bubbles drawn out into a series of 

 parallel canals, all lying in one plane. Any variation of color in dif- 

 ferent portions of the stone also follows the lines of growth in this 

 manner (pi. 2, fig. 3; pi. 3, fig. 1). 



In the artificially produced corundum, on the other hand, although 

 the particles arrange themselves symmetrically, any air bubbles that 

 are entangled in the successive globules remain undisturbed, and 

 appear as naturally spherical bubbles in the finished product; and, 

 moreover, if one globule differs slightly from another in the proportion 

 of chromium, the resulting difference in color follows the form of the 

 mass as a whole, the zones of color being circular (pi. 3, fig. 2). 



As some of the air entangled between the fine particles fed into the 

 blowpipe almost invariably fails to make its escape during the brief 

 fusion, the presence and form of the bubbles is in this way sufficient 

 to identify the artificial process of formation. 



In the great majority of cases examination of the cut stone with a 

 lens is sufficient to decide the point, but in doubtful cases a more 

 minute examination may be made by placing the stone in a little cell 

 filled with highly refracting liquid, in order to secure regular illumi- 

 nation, and examining it under the microscope by transmitted light, 

 when the minutest trace of structure can be detected (pi. 3, fig. 3). 

 In the case of an absolutely flawless stone it would be impossible to 

 decide whether it were natural or artificial, but such stones are so 

 rare that this case is almost theoretical. 



It is claimed in some quarters, it is true, that "experts" can 

 invariably distinguish the artificial product merely by reference to 

 the color, which is said never to be exactly the same as that of the 

 natural stone, much as this latter varies. Personally, however, I am 

 rather skeptical on this point, as one knows that experts claim in a 

 similar manner to distinguish between one species of natural gem 

 stone and another by color alone, and their results are not always in 

 accordance with scientific tests. At any rate such dexterity can only 

 be acquired by a lifetime of specialized experience. 



As I have already indicated, spinels may be produced artificially 

 by the same process as corundum, adding the necessary magnesia to 

 the alumina, and the same remarks apply to the production and 

 identification of this species as to corundum, the artificial stone being 

 identical with the natural in all respects except those to which I have 

 just referred. 



