ROCK CRYSTAL. 47 
In the counties of Cornwall and Derby, in the neigh- 
bourhood of Bristol, and amongst the mountains of 
North Wales, small crystals of this kind are frequently 
found : these are respectively called Cornish, Buxton, 
Bristol, and Snowdon diamonds. We are informed that 
the crevices of some parts of Mont Blanc and the Alps 
contain rock crystal in such abundance as to be per- 
fectly bristled with it. 
Some crystals contain in their substance drops of 
water, or other kind of fluid ; and these, as curiosities, 
are usually sold at a rate considerably higher than 
others. There are in the British Museum specimens 
of crystal which enclose many kinds of foreign sub- 
stances, such as ironstone, needle antimony, and asbes- 
tos (136). 
Various means have been devised for communicating 
colours to rock crystal. If it be heated and plunged 
into a solution of indigo, or copper, it acquires a blue 
colour; or if into a decoction of cochineal, a red colour. 
A clove-brown colour may be given by exposing it to 
.the vapour of burning wood. Artists sometimes com- 
municate beautiful colours to rock crystals, by forming 
them into what are called doublets. Two modes of 
.doing this are adopted. In one, a stone that is bril- 
liant-cut at the top is hollowed underneath, filled with 
the colour that the stone is intended to exhibit, and 
then closed at the bottom by a plate of glass. If this 
kind of doublet be dexterously executed, the deception 
is not easily discovered ; for the whole mass will appear 
of an uniform tint. The second kind of doublet is 
formed by cementing a coloured plate of glass on the 
base of a rose or brilliant-cut crystal : by this the whole 
stone acquires the colour of the plate. 
There are found in nature, many coloured kinds of 
crystal. These are often confounded with precious 
stones ; and, as such, are made into female ornaments 
of different kinds. The following are the principal of 
them. 
