8 GENERAL CHARACTERS OF PRECIOUS STONES 



other groups of minerals which are unimportant from this point of view. Such groups are 

 those of the metallic sulphides (compounds of metals with sulphur), the haloid compounds 

 (combinations of metals with chlorine, bromine, iodine, and fluorine), and the sulphates 

 (compounds of sulphuric acid). Although these three groups may include minerals which 

 are occasionally used as ornamental stones, we find none possessing the essentials of a gem 

 to any marked degree. 



The group containing the silicates is again an important one, for it embraces the 

 emerald, garnet, chrysolite, topaz, and many other precious stones. Tourmaline may be 

 mentioned as an example of the few gems belonging to this group, the chemical composition 

 of which is specially complex. 



Of the other divisions of the mineral kingdom there remains only to be mentioned that 

 in which the phosphates are placed. This division contains only one gem, the turquoise. 

 This important and valuable stone, which is composed of phosphoric acid combined with 

 alumina and water, is remarkable, inasmuch as it is the only costly stone which contains 

 any considerable amount of water as an essential constituent. 



The ornamental stone malachite may be mentioned here as being the representative of 

 the carbonates or compounds of carbon dioxide, and at the same time as containing a con- 

 siderable amount of water as an essential constituent. 



To identify any given stone and to determine the mineral species to which it belongs, 

 a chemical analysis is often desirable and, in some cases, essential. Since this method 

 involves the complete destruction of the substance experimented on as such, it is obviously 

 of very limited application in the determination of precious stones of great intrinsic value. 

 In the case of uncut stones a chemical analysis may be made of detached fragments. But 

 with cut and polished stones, not only is a complete chemical analysis impossible, but the 

 mere testing with acid must be avoided. 



B. CRYSTALLINE FORM. 



Most chemical compounds, including the majority of minerals, frequently occur as solid 

 bodies bounded by plane faces. These shapes have been assumed on the solidification of the 

 substance, and are due to the internal molecular forces exercised by the substance, and not 

 to any external influence. Such definite shapes are known as crystalline forms, and sub- 

 stances occurring in this condition are said to be crystallised. With very few exceptions 

 all precious stones are crystallised. Diamond, ruby, sapphire, emerald, topaz, &c., occur 

 naturally as crystals of the finest development. Only a few, of which the most important is 

 opal, are not bounded by the plane faces characteristic of crystallised substances, but occur 

 only as irregularly-shaped masses. Such substances without definite external form are said 

 to be amorphous. 



Crystallised bodies, therefore, differ from amorphous bodies in that on solidification they 

 assume a regular form bounded by plane faces, which is the outward expression of internal 

 molecular forces. Certain peculiarities in the physical characters of crystallised bodies, 

 which are absent in amorphous substances, are also due to these internal molecular forces. 

 Thus it is still possible to distinguish a crystallised from an amorphous body, even though 

 the characteristic regular boundaries of the former should happen to be absent. 



The absence of the regular boundaries of a crystallised substance may be due to one or 

 more of a variety of causes. Their free development may have been hindered by external 

 conditions ; as, for instance, when a substance crystallises in a confined space where free 



