I. NATURAL CHARACTERS AND OCCURRENCE. 



A. CHEMICAL COMPOSITION. 



Precious stones in their general chemical relations do not differ essentially from other 

 minerals. They are composed of the same chemical elements, which are combined together 

 according to the same Jaws. At one time, however, it was believed that, since the 

 characters of precious stones were so remarkable, their chemical composition must also be 

 unique. Hence it was assumed that all precious stones contained a rare and precious earth 

 as a fundamental constituent. More exact chemical investigations have shown, however, 

 that the constituents of the rarest of precious stones are frequently very common sub- 

 stances, such as carbon or alumina. The precious metals — gold, platinum, &c. — never enter 

 into the composition of gems, and the rare elements very exceptionally so. As examples 

 of such occurrence may be mentioned the element zirconium, present in zircon, and the 

 element beryllium, present in emerald, aquamarine, and a few other rarer stones. 



The chemical composition of different stones varies considerably in complexity. While 

 it is very simple in some, in others it is complicated by the presence of numerous con- 

 stituents. In the case of the diamond, the chemical composition is very simple ; this, the 

 most important of gems, consists solely of the common and widely distributed element 

 carbon. The carbon of diamond, however, is endowed with special properties, and differs 

 very widely from graphite, the other crystallised modification of carbon, and from coal, 

 which consists largely of carbon. Among gems, the diamond stands alone in the simplicity 

 of its chemical composition. 



At least two, and in the majority of cases a number of, elements enter into the com- 

 position of all other precious stones. The rarest and most costly of all stones, the red 

 ruby, contains only two elements ; and the blue sapphire is identical with the ruby in 

 chemical composition, differing from it only in colour. The two elements of which the 

 ruby and sapphire consist are aluminium and oxygen. The former, an important con- 

 stituent of clays and other widely distributed minerals, is a metal which in recent years has 

 become of great importance in the arts and manufactures ; the latter is an important con- 

 stituent of atmospheric air. The combination of aluminium and oxygen, known as oxide 

 of aluminium, or alumina, is an essential constituent of many other valuable gems. Rock- 

 crvstal, amethyst, agate, opal, and other stones also consist of a simple oxide, the oxide of 

 silicon. This oxide, which is known as silica, is the most important constituent of the 

 earth's crust. Zircon, spinel, and chrysoberyl furnish examples of slightly more complex 

 oxides. 



While the group containing the oxides furnishes so many important gems, there are 



