Chap. 14- ] fke Tourmalin and Opal. 87 



eight-fided pyramid, terminating by four faces on each 

 end. In general their luftre is lefs than that of other 

 gems, as v/ell as their hardnefs, which yields to the 

 file, though they fometimes ftrike fire with fleel. Their 

 colour proceeds from the iron which they contain; 

 and, according to M. de Sauflure, even the fineft ori- 

 ental garneto attract the magnetic needle at a fmall 

 diftance. The Syrian garnet is the fineft, and moil 

 eibeemed : ic is of a fine red, inclining to a purple co- 

 lour, very diaphanous, but lefs brilliant than the oriental 

 amethyft. 



9, The tourmalin has lately been brought much 

 into notice by its remarkable property of becoming 

 electrical, in confequence of the fimple application of 

 heat. I fit is immerfed in hot water, in which procefs 

 no friction can be luppofed to happen, after it is re- 

 moved it gives figns of contrary electricity at the two 

 oppofite ends of its priiinatic form. 



The oriental tourmalins are found in the ifland of 

 Ceylon : they are tranfparent, of a dark brown yellow. 

 From Brafil they are for the moft part green ; but 

 there are allb fome red, blue, and yellow. From Tyrol 

 they are obtained of fo dark a green, as to appear 

 opakc. They are alfo found in the mountains of Old 

 Caftile in Spain. 



10. The opal is a moft beautiful ftone, from the 

 changeable appearance of its colours by refraction 

 and reflection. There are two kinds > one of which, 

 the opal of Nonnius, appears olive-coloured by reflec- 

 tion, and then appears to be opake ; but when held 

 againft the light, it is found to be tranfparent, and then 

 appears of a fine red colour. The white opal, of a 

 glafs-like complexion, throws out green, yellow, purple, 

 and blueilh rays j but it is of a reddiih, or rather flame-. 

 colour, when held againft the light. 



G 4 The 



