TOPAZ: PHYSICAL CHARACTERS 329 



not be allowed to fall or to be jarred in any way; and when undergoing the process 

 of cutting great care is necessary, since the perfect cleavage renders the stone liable 

 to chip at the edges and to become fissured or broken by the jarring of the grinding 

 disc. 



The hardness of topaz is represented by 8 on Mohs' scale. It scratches quartz with 

 ease, but is itself readily scratched by corundum. Of the minerals hitherto considered, it 

 is surpassed in hardness only by diamond, corundum, and chi-ysoberyl. On account of its 

 hardness it takes a good polish and exhibits a brilliant lustre, which is also to be seen on 

 the natm-al crystal-faces. 



Topaz is a comparatively heavy mineral ; its specific gravity, determined on different 

 varieties, ranges from 3'50 to 3'57. Colourless topaz is often rather heavier than the 

 coloured varieties ; its specific gravity has been determined at from 3"53 to 3'56. This 

 value is almost exactly the same as that of diamond, so that it is impossible by the 

 density alone to distinguish between a diamond and a colourless topaz, which, when cut, 

 have a certain resemblance. The specific gravity of the reddish-yellow topaz of Brazil 

 (Plate XIII., Figs. 2 and 2a) and Asia Minor has been determined to be 3-50 to 3'55, 

 while that of the greenish-blue from Nerchinsk is 3'53. Such small variations in the 

 specific gravity are due to differences in the chemical composition depending upon the 

 replacement of fluorine by hydroxyl. According to some determinations the specific gravity 

 may vary between 3'4! and 3'6 ; this greater departure from the mean value, 3'5, is 

 probably due either to impurity of material or to inaccuracy of determination. 



When rubbed, topaz becomes strongly electrified and capable of attracting to itself 

 any light bodies such as shreds of paper. Some topazes possess this property in a more 

 marked degree than do others ; thus, for example, those from Schneckenstein in Saxony 

 acquire a chai-ge of electricity when merely rubbed between the fingers, while in the case of 

 certain Brazilian topazes a pressure of the fingers, exercised in the direction of length of 

 the prismatic crystal, is sufficient; also, when heated and allowed to slowly cool, topaz 

 becomes electrified and acquires a greater charge than would any other precious stone under 

 similar circumstances, with the exception of tourmaline ; on this account it is said to be 

 pyroelectric. The charge may be retained for thirty hours or more after the stone has 

 cooled down to its original temperature. This phenomenon, under certain circumstances, 

 affords a means by which topaz may be distinguished from other stones which it may 

 resemble in general appearance. 



In the blowpipe flame topaz does not fuse, but becomes cloudy and opaque owing to 

 the loss of water and of fluorine ; coloured stones lose their colour. Acids, whether hot or 

 cold, have no action on topaz. 



More important than any other feature, however, are the optical chai'acters of topaz, 

 that is to say, its behaviour towards light. In this connection we may begin by 

 {listino-uishing between cloudy and opaque topaz, the so-called "common" topaz, and that 

 which is clear and transparent, " precious " topaz. The former, besides being opaque, is 

 usually nondescript in colour, so that it is unsuitable for gems. A variety of common topaz, 

 known as pyrophysalite, occurs as large crystals in granite near Fahlun in Sweden ; another 

 varietv, known as pycnite, is found as columnar aggregates in the tin mines of the 

 Erzgebirge between Bohemia and Saxony. Our attention must be devoted, however, not to 

 these varieties of common topaz but to precious topaz, the transparency, colour, and lustre 

 of which combine to make it a very beautiful gem. 



Its lustre is of the ordinary glassy or vitreous type ; on the cleavage face, however, it is 

 ijearly. The brilliant lustre of the natural crystal-faces has been already mentioned, and is 



