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 (pl. 2, fig. 3; pl. 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 (pl. 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 celi 
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 (pl. 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. 
