32 GENERAL CHARACTERS OF PRECIOUS STONES 



usually be sufficient to determine the identity of the stone. Stones which are harder than 

 topaz are but few in number; they are corundum (including ruby and sapphire), chryso- 

 beryl, and diamond, the hardest of all stones. These stones stand alone in their power of 

 scratching topaz ; they may be readily distinguished from each other by the specific gravity 

 or other determinable characters, as in the case of the few stones which do not scratch glass. 



Determinations of hardness must be performed on cut stones with the greatest care, 

 for it is possible that corners of the stone may chip off even when being pressed against a 

 softer stone, and especially so when the cut stone possesses a good cleavage as in topaz or 

 diamond. The loss of a corner would not be serious in a rough stone, since, in the process of 

 cutting, broken edges are removed ; but it would be fatal to the perfection and beauty of a 

 cut stone. This test of hardness then, though useful in the case of rough stones, must only 

 be used with caution in the case of valuable cut stones. 



In place of the scale of hardness, the use of which has been just explained, the dealer 

 in precious stones more frequently uses other instruments, and specially a hard steel file. 

 This easily scratches minerals with a hardness of 5, and only slightly those with a hardness 

 of 6, giving more or less powder according as the hardness of the stone is less or greater. 

 Quartz is of about the same hardness as hardened steel of good quality, of which the file 

 should be made. Stones with a hardness of 7 are therefore only with difficulty marked by 

 the file, while harder stones will rub and polish the file, which will leave a shining, metallic 

 mark on the stone. An approximate idea of the hardness of a stone may be obtained from 

 the pitch of the sound emitted when the file is rubbed on the stone. Provided that stones 

 similar in size are used for testing, then the harder the stone the higher the note emitted. 



In the case of cut stones the file is too clumsy an instrument, and the practical jeweller 

 uses in its place a pencil of very hard steel furnished with a sharp point. This pencil 

 scratches felspar easily and glass still more easily ; it scarcely touches quartz, however, and 

 has no effect on harder stones. The girdle of a cut stone is a suitable part on which a trial 

 of its hardness may be made ; it being by the girdle that the stone is fixed in its setting, a 

 small scratch in this region is unnoticed. The steel pencil is especially useful in distin- 

 guishing genuine precious stones from their softer glass imitations, since the former cannot 

 be marked by it, while the latter are scratched with ease. As before mentioned, however, the 

 greatest care is needed in testing cut stones, especially the transparent kinds, so that even 

 this more refined method has certain limitations. 



The hardness of a stone is naturally a question of great importance in the process of 

 cutting ; the material of the grinding disc and the grinding powder, to be described later, 

 must be chosen according to the degree of hardness of the stone. When worked under 

 similar conditions, with the same kind of abrasive material, the harder the stone the longer 

 and more difficult will be the process of grinding. As a rule also, the harder the stone the 

 sharper will be its edges and corners when cut, and the more susceptible will be its faces of 

 a brilliant polish. The edges and corners of softer stones are much less sharp, and con- 

 sequently these stones have a less pleasing appearance. A high degree of hardness is thus 

 not only essential for the preservation of the, beauty of a stone, but is also one of the 

 properties on which its beauty depends. 



From the time required for grinding a facet on a stone, it is possible to form an estimate 

 of the hardness of the stone on this facet. It not infrequently happens that a stone can be 

 more easily and quickly cut in certain directions than in certain others ; moreover, not only 

 do the different natural faces of a crystal vary in hardness, but the hardness on any face is 

 not identical in all directions. The hardness of a crystal, therefore, like the other physical 

 charactei's, varies with the direction. 



