3 6 Dr. Eqfon on Cryjiallizatidn. 
cil'cum(lances, which need not be mentioned. * 
Thus, for inftance, an admixture of metals may 
give to the cryftal different colours, fuch as 
the ruby, the emerald, the topaz, or the 
amethyftjf ancl 3 w ^ en the cr yft a l ls perfedly 
pure, it conftitutes the colourlefs rock cryftal. 
Is not this hypothefis ftrongly fupported by 
the diftipation of the colours of the precious 
Hones ? And is not the reftoration of thofe co¬ 
lours a ftriking proof, that this hypothefis is 
founded in nature ? 
Three principles, at leaft, feem to enter into the 
compofition of precious ftones, viz. vitrifiable 
earth, the metallic calces on which their colours 
depend, and the particular matter determining 
the figure into which they cryftallize ; the variety 
of which might be inftanced by feveral examples. 
The prefence of the principle juft mentioned is 
ftill further indicated by the different degree of 
hardnefs obfervable in precious ftones. The 
rock cryftal, although as colourlefs and tranfpa- 
rent as the pureft diamond, is, neverthelefs, the 
* When cryftals are found irregular and not pure, the 
vulgar expreflion is, that they are not ripe; but the truth 
is, they never could ripen, though left to the end of time, 
on account of fome heterogeneous matters having entered 
into their compofition. 
f Bergman has ftiewn that all thefe colours may be 
imparted to gems by iron only. 
fofteft 
