CATALOGUE OF GEMS. 



531 



a .specific gravity of 3.5 to 3.65; becomes strongly' electric hy friction, 

 heat, or pressure, remaining* in that condition for some time, a charac- 

 teristic so inarlvcd as to ati'ord a vahialile means for its discrimination; 

 a vitreous luster; a strong double refraction: ;uid. in colored kinds, a 

 marked pleochroism. 



The colors of the topaz include wine, am)>er, honey, and straAV-yellow, 

 pale blue to pale green of several shades, grayish, reddish, and white. 

 Colorless specimens, especialh* the rolled pebbles, are peculiarly limpid, 

 hence the name c/outtes d\au. (Fig. 13.) The best colorless topazes 

 have considerable tire. and. when properly cut. exhibit brilliant retlec- 



Fig. 13. 



TUPAZ PEBBLES [GOUTTES D' EAU]. 



Mitchell River, New South Wales. 



Specimen No. 83782, U.S.N.M. 



tions of white light, approximating that of the diamond. The rose- 

 pink topaz is probabl}' not known in nature, the delicate tint being 

 commonh' obtained by heating the yellow or brown colored kinds. 

 The process of " pinking" is quite simple. The selected stone is packed 

 in magnesia, asbestus. or lime, and carefulh' heated to a low red heat, 

 care being taken that the temperature is raised gradually: the stone 

 is then allowed to slowly cool. If the temperature reached has Itecn 

 sufficiently high, the desired rose-petal tint is obtained; if not high 

 enough, a salmon tint: if too high or too long continued, tlu^ color is 

 lost completely. 



