Opal Enlarge
Oct 1 1810 published by Jas Sowerby London
British Mineralogy
CCCLXXVII
Silex quartzum, var. opalinum

Opal

I have given (Brit. Min. 111) a figure and description of the Precious Opal of Cornwall. I now show a variety from Sandy Bræ, brought from thence by Humphry Davy, Esq. of high chemical celebrity. Opals have been divided with some difficulty into several kinds; as The Precious Opal, The Common Opal, Semi- or Halb-Opal, and Wood Opal. The good and bad might have been sufficient distinctions, as the several kinds run into each other. The three first arrange according to their goodness; the Precious Opal having a pearly brilliancy with a fine soft display of most of the prismatic colours, and a brilliant lustre peculiar to itself: the Common Opal has less of this lustre; aud the Halb-Opal borders on it so feebly as to be considered as only partly an Opal: the Wood Opal may have all these varieties, but is infiltrated into the remains of wood retaining its structure. These distinctions therefore do not all belong to the substance, and become incorrect, or mislead. The present specimens are remarkable: the upper one, for a fine somewhat resinous appearance, from a light dull greenish yellow to a resin brown, with a fiery or orange yellow sparkling splendency reflected as the light passes among the flaws.

The upper specimen is in a sort of decomposing Porphyry, spotted and veined with resin-like Opal mostly dull in colour.

The lower figure has something of the appearance of light Burgundy Pitch; and is such as is often found in the ligniform state. I have an example in a foreign specimen mingled with Pitch-stone.

Opals are chiefly Silex with from 5 to 10 per cent, of Water, and, under certain circumstances, are so operated upon, that moisture and change of temperature more or less sudden seem to cause the various appearances; and those that become degraded below the Precious, are less and less pure with various adulterations. Generally found in more or Jess perfect Porphyry.

Analysis of the Opals:
Precious Opal. Common Opal. Semi-Opal.
Silica 90. 93.50 43.50
Oxide of Iron 1.00 47.00
Water 10. 5.00 7.50
Loss 50 2.00
100. 100.00 100.00
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