Chap. 1 4.] G-eneral Properties of Gems-. 9 



tenfe fire ; yet we know that the ruby has been fof~ 

 tened in the focus qf a burning mjrror *. 



Thefe (lon-s all affume different forms of cryftalli- 

 zation, and as this affe6ls the arrangement of their 

 minute parts, it cbubdefs is not without eiTed; on their 

 external appearance. Their value depends on their 

 hardnefs and tranfparency. The degrees of h-irdnefs 

 are i, diamond; 2, ruby; 3, fapphire ; 4, topaz; 

 5, hyacinth ; 6, emerald. 



The garnet differs from all die above, in the fi- 

 liceous earth being more prevalent than die argilla- 

 ceous. 



II. QAURTZ has lefs tranfparency and hardnefs 

 than the precious ftones. The fracture of quartzofe 

 Hones is vitreous, or like glafs, and they ftrike fire 

 with fteel. Heat caufes them to, lofe their hardnefs. 

 and tranfparency, and reduces them to a white opake 

 earth. They are generally cracked throughout, and 

 break irregularly, and into {harp fragments. Melted 

 with alkali they give a more folid and fixed glafs than, 

 any others of the filiceous order. When there' is no 

 interruption to. their natural accretion, their fubftance 

 always cryflallizes into hexagonal prifms, pointed at 

 one or both ends, Thefe occur in clefts, fiffures, and 

 fmall veins in rocks. Quartzofe flones very often 

 contain metals. 



No very remarkable ftones belong to this genus. 

 The varieties are Fat quartz, which is very glofly j it 

 is either colourlefs, or is tinged with white, blue, or 

 violet. Dry quartz, tranfparent, white, or pale green. 

 Sparry quartz, pale yellow, or pale blue. Cryftal- 

 lized quartz is either opake or tranfparent : the tranfpa- 



* Chemical Eflays, Diff. 15. 



rent 



