Siliceous Genus, ii 



nomenon is well explained in the fecond vo- 

 lume of Mr. Bergmans works. 



IV. VARIETY* 

 Onyx. 



This is a very hard ftone of the colour of 

 the human nail, with ftraight or circular 

 zones of another colour ; if the zones them- 

 felves be of different colours, it is more 

 efteemed, and called camehuya : it is almoft 

 opake ; its fpecific gravity is from 2,5 to 2,6 ; 

 it is affected by fluxes as the former varieties. 



V. VARIETY, 



Carnelian* 



Its colour is of different fhades of red ; it 

 is alfo of different degrees of tranfparency 

 and hardnefs : the beft is of an orange or yel- 

 low red, and gives fire with fteel ; the whitifh 

 or yellowifh are too foft to give fire with 

 fteel, and lefs efteemed; its fpecific gravity 

 is from 2,6 to 2,7; it lofes its colour in fire. 

 Fluxes affect it like the above. 



VI. VARIETY. 

 Sardonyx* 



It confifts of chalcedony and carnelian, 

 united either in zones, ftfata, or fpots. 



I 2 SPECIES VI, 



