18 



Case 49. 



Topaz. 



Silicate of alumina with fluoride of silicon : one of the few 

 minerals that contain fluorine. "When perfectly transparent, as 

 it frequently is, topaz is a useful gemstone. It is very hard 

 (H=8) scratching glass with ease. While generally colourless 

 (9219) or of a yellowish tint, topaz is sometimes blue (8029 and 

 others) or pink. As pebbles, it is common in many river-drifts, 

 particularly those of New England (2317, 8029). The pebbles 

 very much resemble quartz, but can be distinguished — 



(«) by splitting with perfectly even cleavage-flakes; crystals 

 purposely cleaved by tapping with a hammer are 

 placed here, 



(h) by greater density, topaz sinking rapidly in heavy liquids 

 (see page 9.) 



Crystals of topaz are not uncommon, and can be readily recog- 

 nised by their characteristic form (9219, and a blue crystal, the 

 faces of which have been marked with their crystallographic 

 symbols). 2380, a large blue crystal from New England, shows 

 the terminal faces of topaz well. 



Like beryl, tourmaline, fluor, and tinstone, topaz generally 

 originates in granitic rocks, and is frequently associated with 

 those minerals. 9082 and others, from the Emmaville emerald 

 mine, illustrate this. A few cut stones are exhibited. 



Crystals of Topaz. 



