ARTIFICIAL PRECIOUS STONES—-HEATON. 231 
as this involves the destruction of the stone it can not be offered as a 
practical test. 
Opal consists essentially of what is known as colloid silica, that is, 
silica in the amorphous state and combined with water. The play 
of color one associates with it is entirely an optical effect, due to an 
accidental structure of the stone, which is permeated by a number of 
minute fissures between which a thin film of air penetrates, the 
extreme thinness of this film causing the optical effect known as 
interference. If a piece of opal is powdered it is no longer colored, 
as would be the case with a ruby or sapphire, but yields a dirty white 
powder, and generally a specimen of opal, as found, only shows the 
structure in parts, the remainder being dull and lusterless like flint. 
This peculiar structure is, moreover, by no means confined to opal, 
but may occur in any mineral deposited under similar conditions. In 
the mineral known as lumachello, or fire marble, for example, the 
same effect is seen in a limestone. But opal is the only mineral which 
combines this structure with sufficient durability for use as a gem 
stone, and in this connection it should be remembered that, as a 
matter of fact, it only just possesses sufficient hardness for this pur- 
pose, and is one of the softest and least durable of all the precious 
stones. This fact, combined with the fragility consequent upon its 
structure, has involved the opal in a mass of superstition and romance 
from time immemorial. 
Although it has this unfortunate drawback, opal is, at any rate 
in my estimation, the most beautiful of the precious stones, and when 
one appreciates the reason of its beauty it will be readily understood 
that its artificial production, or even successful imitation, presents 
almost insuperable difficulties. 
It is true that a somewhat similar play of color can be imparted 
to glass by rendering it translucent by a slight addition of arsenic or 
tin in the making, and by etching the surface in various ways, and 
such iridescent glasses are often found naturally as the result of 
decomposition, but this is merely a surface effect, and such speci- 
mens can not be cut to advantage; moreover, they lack the beauty 
caused by the fire permeating the entire substance of the gem. The 
opal ranks with the diamond, therefore, in resisting attempts at 
artificial production, and is even superior to it in that it can not be 
really successfully imitated. 
I come finally to the pearl. This, of course, differs from all other 
precious stones in being entirely of organic origin. The peculiar 
luster of the pearl, like the color of the opal, is due rather to its 
structure than its composition. It is formed in the oyster by the 
deposition of successive layers of calcium carbonate round some 
central object, and consists of an innumerable number of thin over- 
lapping laminee of the ervstailine variety of this substance known as 
