178 PRECIOUS STONES.
stalactitic masses. Precious Opal more often is found mixed with matrix and forming disseminated patches and veins.
The hardness is 5-5 to 6'5.
The
origin seems very similar to that of many other forms of
silica—deposition in cracks and cavities in rock from watery solutions
of silica, usually percolating downwards, as described previously. It
is usually met with in decomposing volcanic rocks, hut may he found in
any silica containing rock. It is associated with Quartz, Chalcedony,
and other modifications of silica as a rule.
In chemical composition it is a hydrated oxide of silicon, Si02, n.H2O,
the amount of water varying, so that one may almost say there is a
transition from Opal to Chalcedony. The water percentage varies from
0.1 to 10.0 per cent, or more. Heating drives off this water and ruins
the gem.
Varieties:—
1.
Precious Opal—that showing the fine play of colour described above.
According to the form in which the colour appears (the "pattern" of the
Opal), it is subdivided into
a. Harlequin Opal ,that in which the light appears in
small angular patches ;
b. Pin Point Opal, in which the points of light are very
minute;
c. Flame Opal, with the colour in streaks;
d. Gold Opal, showing a yellowish light over a large
area
Precious
Opal in Pliny's time came solely from India, though none is known from
that country now. Even early in the seventeenth century India still
produced the
