90 Qiiartx, Rod Cryftal, fc. [Book VI. 



rent and dark- brown kind is called fm ok y- topaz; the 

 yellow, blue, green, and red, falfe gems ; and the co- 

 lourlefs, rock cryftals ; when milky, milk cryftals, and 

 pebbles. .Quartz is alfb found combined with iron 

 and copper ; with the former it conftitutes a black 

 calx, with the latter a red calx. 



III. FLINTS are more uniformly folid, and not fo 

 much cracked in the mafs, as quartz ; and are more 

 pellucid than jafper. They are better for making 

 glafs than the jafper, but not fo good as quartz, and 

 feem in moft refpecls to be of an intermediate nature 

 between thefe ftones. Flint often fhews evident marks 

 of having been in a foft and tough (late, like glue or 

 jelly. 



The feveral varieties of flints have obtained more 

 ' diftindi names, from the variety oi their colours, than 

 from any real difference in their fubftance ; but th^fe 

 are dill proper to be retained, as the only names 

 by which jewellers and others are ulcd to diftinguifh 

 them. 



i. Jade, lapis nephriticus. This (tone feels unc- 

 tuous to the touch, but is fo hard as to ftrike fire with 

 fteel, and is alfo fcmipellucid. Thefe latter circum- 

 fiances fufficiently denote its flinty nature though its 

 undhiofity has induced fome mineralogifts to think 

 that it ought rather to be referred to the argillaceous 

 or iv.agnefian orders. It is not hardened by the heat 

 of the furnace, but it melts by the folar heat, in the 

 focus of a burning mirror, into a green glafs. That 

 called by the name of circuracifion-ftone, which comes 

 from the Amazon river, melts more cafily by the 

 concentrated rays of the fun into a brown opake glafs. 

 The colour of thefe (tones is either milky, or different 

 fhades of green. Thofe of a grey, olive, or yellowifh 



colour> 



