8. Jasp-Opal is a form containing sufficient diffused impurity (often an iron compound) to render it opaque.
9.
Wood Opal consists of the fossil remains of wood infiltrated with
hydrous silica. When cut and polished it shows all the detailed
structure of the wood very clearly. It is found near Hobart Town in
Tasmania, Kremnitz in Hungary, and many other places.
10.
Hyalite, or Muller's Glass, is an absolutely colourless and usually
transparent Opal occurring in small botryoidal masses resembling beads
of fused glass. It resists solution by alkalies more than other forms ;
it is found at Kremnitz in Hungary and in Bohemia, amongst other places.
11.
Silicious Sinter is a more porous form, chiefly of interest from
its origin in deposition from uprising heated water in fumaroles and
hot springs.
The
applications to which Opal is put are all those of ornament, and these
have been already indicated. All kinds are cut in thin slices, usually
with a curved upper-surface (en cabuchon), except the Fire
Opal, which may be cut in a deeper form and facetted. The value depends
very largely on the quality of the colour and on the pattern. The
favourite type is a Harlequin Opal showing bright green and crimson
flashes; such a stone of one carat may fetch over £2. More recently Opal cut in the matrix has been fashionable, though how long it will remain so is uncertain.
Opal
is rarely imitated, but sometimes glass imitations are mounted with
foil in a closed setting to give the effect of play of colour; such an
imitation can be detected at once by the inferior hardness of the glass.
