Opal, the hydrous form of silica, containing from 3 to 13 per cent. of water. It is amorphous, having apparently consolidated from a gelatinous state, and, from the unequal strains induced in this consolidation, behaves with reference to polarised light like a doubly-refracting crystal. Its hardness is 5-5 - 6-5, and its specific gravity 19 - 23. It may be opaque or subtranslucent, one of the latter varieties, precious or noble opal, exhibiting a, beautiful play of iridescent colours. These are explained as due to interference produced by thin included films. It occurs, in veins in trachyte, in Hungary and Honduras; and in ironstone nodules in Queensland; and is a valuable gem. Hyalite is a transparent, colourless variety; fire-opal, a transparent, ued, or yellow one; common opal gives no play of colour; and menilite or liver-opal, from Menilmontant, near Paris, is opaque and brown or grey. Of the less translucent varieties or semi-opal, the most interesting are wood-opal, replacing particle for particle the tissues of fossil plants, and geyserite or siliceous sinter, deposited by the heated waters of geysers.