PRECIOUS STONES. 177
The
outstanding feature of Opal is its colour—at any rate, so far as the
variety used as a gem stone (Precious Opal) is concerned. What the
colour is due to has never been quite ascertained, though there is no
doubt it is an optical effect; it has been suggested that minute
cavities cause refraction and reflection (Brewster); also that it is
due to interference of light in cracks; more recently Behrends has
ascribed it to the existence of numerous thin lamellae, which have at
one time been parallel, but later bent and cracked into a curved form.
The colours shown in Precious Opal are remarkable for their intensity,
and may be likened to the light emitted by some of the double stars, or
to the colours seen in the feathers of certain birds. Thus brilliant
and pure greens, vivid crimsons, electric-blue are seen, with often a
dominant soft blue; sometimes a rich violet or a sherry yellow appear,
very rarely rose-red and black. Owing to the substance being
sub-transparent, and of a pale amber colour by transmitted light, an
Opal often shows a blending of this yellow light transmitted through
the stone with the optical colours, giving an appearance known as
"opalescence." Other varieties of Opal may show different colours, as
white, yellow, brown, grey, green, etc., quite apart from any play of
colour.
The lustre varies from vitreous to resinous, or rarely pearly; sub-transparent to opaque.
Refraction is normally single, n,. = T442—P446 ; but anomalous double refraction is sometimes seen through internal strain.
The specific gravity is 1.9—2.3, purer forms about 2.19— 2-2. It has a conchoidal fracture, and gives a white streak.
It is always amorphous, and occurs in reniform or
p.s.
K
