64 HYDROPHANOUS OPAL. 
them a distinguished rank among gems. Notwith- 
standing this, they are but ill suited to the purposes 
of jewellery, on account of their softness, their great 
frangibility. and their sometimes splitting on a sudden 
change of temperature. They are usually set without 
bottoms; but sometimes with a black * bottom, and 
sometimes with a foil of red, blue, or gold colour. 
Their value is such that a fine oriental opal is considered 
worth about twice as much as an oriental sapphire of 
the same size. By the Turks they are so peculiarly 
esteemed, that a fine opal of moderate size has some- 
times been sold at the price of a diamond. The esteem 
in which they were held among the ancient Romans was 
such, that Nonius, the Roman senator, is stated to have 
preferred banishment to parting with a favourite opal 
which Mark Antony was anxious to possess. 
In the abbey of St. Denys, near Paris, there was 
formerly a curious ancient opal which was green on the 
outside, and, when viewed against the light, exhibited 
a fine ruby colour : and in the Imperial Cabinet at 
Vienna, there are two pieces of opal, from the mines in 
Hungary, one of which is about five inches long, and 
2- inches broad ; and the other the size and shape of a 
hen's egg. BotlT these stones exhibit a very rich and 
splendid play of colours. 
In the purchasing of opals great caution is requisite, 
as fine glass pastes have not unfrequently been sub- 
stituted for them, and sold at enormous prices. 
103. HYDROPHANOUS OPAL, or OCULUS MUNDI, is 
a kind of opal ', the distinguishing characteristic of which 
is, that it gradually becomes transparent, and exhibits a 
beautiful play of colour after being immersed in ivater. 
It is either of a whitish brown, yellowish green, milky 
grey, or yellow colour, and opaque ; and, when touched 
by the tongue, adheres to it. 
The name of oculus muntii has been given to these 
stones from an internal luminous spot, which changes 
