408 
OPAL. 
According to Patrin, it is not the little clefts 
in the opal that alone cause the different colours 
which are reflected from its surface ; for he tells 
us that, on exposing several opals immediately taken 
from the mine, they will at first appear equally 
milky, and exactly alike in texture and fineness. 
All will become Hawed in the same manner by the 
action of heat ; nevertheless, some will acquire the 
greatest beauty, while others will hardly possess a 
single coloured ray. 
The white and milky opals which reflect no co- 
lour, are called moon stones : they are very common, 
and but little esteemed. 
