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Sec. III, Ch. 10: The Opal

Sec. III, Ch. 10: The Opal Page of 366 Sec. III, Ch. 10: The Opal Text size:minus plus Restore normal size   Mail page  Print this page
The Opal.                                   217
Semi-Opal. Certain Opals from Zimapan, in Mexico, possess a bright orange-red tint, and are used to a limited extent as an ornamental stone under the name of Fire-Opal. But the rare and beautiful variety, which is familiar to every one by its unique colours, is distinguished scien­tifically as Precious or Noble Opal, though known to the jeweller simply as " Opal."
The value of the Opal lies in the depth and variety of the rainbow-like tints which it exhibits. This colour is not due to any pigment in the stone, but is an optical phenomenon, probably the result of a number of fissures which traverse it, the light being decomposed by the delicate striations on the walls of these microscopic crevices, thus giving rise to "diffraction." The optical properties of the Precious Opal have frequently been made the subject of study by physicists in this country, notably by Sir David Brewster, Sir William Crookes, and Lord Rayleigh.
In some varieties the colours are more or less evenly distributed, and one set of shades will predominate in one part of the stone, and other colours in another part ; or the distinct tints will run in parallel bands. In other specimens the colours are made up of small regular angu­lar patches of every hue, and these polychromatic stones are known as Harlequin Opals. Recently I have found a piece with a luminous ray running down the middle, as in a cat's eye, and I have therefore called this Cats-Eye Opal.
The Opal is a non-crystalline mineral. When first taken out of the earth it is not very hard, but subse­quently, by exposure to the air, its hardness is increased : nevertheless, it always remains a soft stone compared with other gems. Before the blow-pipe the Opal is infusible, but the water driven off by heat renders it opaque. It
Sec. III, Ch. 10: The Opal Page of 366 Sec. III, Ch. 10: The Opal
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