1 1 6 TLlements of Mineralogy. 



SPECIES VI. 



Siliceous Earth perfectly united /0, from an 



equal to 3 times its 'weight of Ar gill, and 



from 7 to an equal weight of Calcareous 



Earth) together 'with from ^ to an equal 



weight of Iron. 



Precious Stones of the frft Order. 



I derive the denominations of thefe ftones 

 from their colour and hardnefs, and not from 

 their hardnefs fingly, as Jewellers moftly do, 

 as this property, though it contributes to 

 their fplendor and value, has but little or no 

 connexion with their compofition. I alfo omit 

 many fub- denominations of each fort of thefe 

 ftones, which are to be found in treatifes ex- 

 prefsly written on that fubjeft. The fpecific 

 gravity of all of them exceed 2,76, whereas 

 that of coloured cryftals never does. Their fin- 

 gular luftre and tranfparency are well known. 



I. VARIETY. 

 Red. 



Ruby, Oriental. 



It is commonly found cryftalized in an 

 odtohedral form ; its texture is foliated; its 



hardnefs 



