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, anc 

 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 lamina of the crystalline variety of this substance known as 



