OPAL. 63 
101. MOCHOA STON-E is a kind of agate, which has on 
its surface the resemblance of moss ; and this so nearly 
approaching a natural appearance, that some persons 
have actually supposed it to be occasioned by a con- 
densation of moss into stone. Its name is derived from 
mocks* the German word for moss. 
These stones are used for several ornamental pur- 
poses ; and are not unfrequently imitated, by spreading 
a solution of copper in nitric acid or aqua fortis (30) 
over the surface of a plain agate, and then setting a 
small iron nail on its head in the middle. The acid 
unites with the iron, and deposits the copper in beautiful 
ramifications from the centre. The nail must then be 
^removed, and the surface carefully washed by dipping 
the stone into warm water. Afterwards, on the appli- 
cation of a moderate heat, the copper becomes black. 
As, however, the deposition is merely superficial, it re- 
quires to be covered with glass, to preserve it from 
injury. 
102. OPALS are a semi-transparent kind of stones, which 
have a milky cast, and, when held betwixt the eye and the 
light, exhibit a changeable appearance of colour. 
They are always found vi a shapeless or massive stale, are 
brittle, and considerably less hard than most other precious 
stones. 
The only opal mines in the world are those of Hun- 
gary. About four centuries ago, opals were obtained, 
in such abundance, from these mines, that upwards 
of three hundred persons were employed in them. 
They still produce opals, some of which are so valuable 
as to pass, in commerce, under the appellation of 'oriental 
opals, whilst others are so poor as to be of no value 
whatever to the jeweller. Opals are also found in other 
parts of Europe ; and in the island of Sumatra and 
several parts of the East Indies. 
Few precious stones are more beautiful than opals. 
Their elegant play of colours, brilliant blue, green, 
red, and yellow, variously modified, has procured for 
