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opal

(Science: chemical) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.

The precious opal presents a peculiar play of colours of delicate tints, and is highly esteemed as a gem. One kind, with a varied play of colour in a reddish ground, is called the harlequin opal. The fire opal has colours like the red and yellow of flame. Common opal has a milky appearance. Menilite is a brown impure variety, occurring in concretions at Menilmontant, near paris. Other varieties are cacholong, girasol, hyalite, and geyserite.

Origin: L. Opalus: cf. Gr, Skr. Upala a rock, stone, precious stone: cf. F. Opale.