HARDNESS 



33 



These differences in hardness, however, are usually very small, and require for their 

 detection an instrument of special construction capable of precise nieasurement ; such an 

 instrument is known as a sclerometer. The somewhat rough method of scratching, described 

 above is useless for the detection of such small differences in hardness ; it is applicable, 

 however, in the case of kyanite. The hardness in different parts of a crystal of this mineral 

 varies between that of apatite (5) and that of quartz (7). No other mineral used as a gem 

 shows variation in hardness between such wide limits. . Certain differences in hardness, shown 

 by different specimens of the same mineral species, may be attributed in part to the fact that 

 the hardness has been determined in different directions ; this difference in crystallised precious 

 stones, however, is so small as to be of little importance. In amorphous stones, such as opal, 

 and in glasses, the hardness, like all other physical characters, is the same in all directions. 



Finally, it must be noted that the hardness of a mineral is distinct from its frangi- 

 bility, the quality on which depends the ease or difficulty with which a stone is broken by 

 a blow from a hammer. The frangibility of a stone depends not only on its hardness, but 

 also, and to a great extent, on the quality of cleavage it possesses. Contrary to popular 

 opinion, the diamond, in spite of its enormous hardness, is very brittle and can be easily 

 broken to pieces. Certain peculiarities of structure greatly diminish the frangibility of some 

 minerals ; this is especially the case in those of which the structure is that of a matted 

 aggregate of very fine fibres or needle-shaped crystals of microscopic size. An example of 

 such a mineral is furnished by nephrite (jade), which, although its hardness is scarcely equal 

 to that of felspar, offers a very great resistance to the hammer, and can only be broken with 

 considerable difficulty. Such substances are described as being tough, while those which 

 are easily frangible would be described as brittle. A high degree of brittleness is not 

 a desirable quality in a precious stone, since the stone is liable to be broken in use unless 

 special care is taken. 



In the following table are given all the more important minerals, which may be used as 

 gems or as ornamental stones, arranged in order of hardness, from the softest to the hardest. 

 The numbers refer to their degrees of hardness on Mohs' scale : 



Precious Stones arranged according to Hardness. 



Demantoid 



Idocrase 



Olivine 



Chalcedony (agate, carnelian, &c.) 



Axinite 



Jadeite 



Quartz (rock-crystal, amethyst, citrine, 

 jasper, chrysoprase, &c.) 



Tourmaline 



Cordierite 



Garnet (red) 



Andalusite 



^ Staurolite 



' Euclase 



Zircon 



Beryl (emerald, aquamarine) 



Phenakite 



, Spinel 



Topaz 



Chrysoberyl 



Corundum (ruby, sapphire, &c.) . 



Diamond 



6J 



6f 



^ 



n 



7f 



9 

 10 



