chinks, in which the air immediately takes its place. This phenomenon is very clearly exhibited in the Hungarian opals.
It
only preserves its brightness in a temperate atÂmosphere ; and although
sometimes it is placed in the sun that its iridescence may be
increased, it spoils if left in it too long. In such a case, the
bubbles of air contained in the internal chinks evaporate, and with
them the colours depart ; the same thing happens in an intense and
prolonged frost.
Although,
mineralogically, the opal may be divided into seven varieties,
according to the countries where found, viz., Arabia, Ceylon, Hungary,
Iceland, Scotland, Ireland, and Mexico, yet it has been generally
agreed to distinguish it into three principal kinds, namely, the noble
or Oriental opal, the fiery red or Mexican opal, and the common opal.
The noble or Oriental opal is called harlequin, on
account of its many colours, which shine in triangular reflections. Its
hardness, and the softness of its colours are remarkable. Its specific
gravity is 2-95. When analysed, it yields
