OPAL. 407 
volcanic remains, others argillaceous rocks, and a 
third class decomposed porphyry. 
Many countries of the world yield this beautiful 
stone, particularly Saxony, Bohemia, Iceland, Ara- 
bia, and the Indies ; but the most beautiful, or 
those which have obtained the name of oriental 
opals, come from Hungary. One of the most re- 
markable mines of opal in this last country is situ- 
ated in a hill at the foot of the mountains near the 
village of Czernizka, some miles from Eperies. The 
lava, which serves as a bed for the opals, and which 
covers one part of the hill, is two or three fathoms 
thick ; and it is observed that the finest opals are al- 
ways found near the surface, and that they are some- 
times turned up by the plough. 
The beds which contain the opals are occasionally 
so penetrated by moisture, that the opals them- 
selves are soft enough to break between the fingers ; 
but when they have been exposed to the sun for a 
few days they recover their usual hardness, and re- 
assume their beautiful colours. In drying, the stone 
contracts a number of little fissures, or clefts, by 
which the rays of light are reflected and decom-. 
posed so as to produce all those varied tints for 
which the opal is so eminently distinguished ; and 
it should be noticed that these tints vary according 
to the direction in which the stone is viewed. Thus* 
for example, when an opal reflects the blue or green 
rays from its surface, if held up to the light and 
examined in this situation, it will often appear 
throughout of the colour of fire. 
