270	OPALUS.
	Pliny's poetical description of his Opalus sufficiently proves that he is speaking of the beautiful gem still bearing that name. " Made up of the glories of the most precious gems, to describe it is a matter of inexpressible difficulty : there is in it the gentler fire of the Ruby, there is the brilliant purple of the Amethyst, there is the sea-green of the Emerald, all shining together in an incredible union. Some aim at rivalling in lustre the brightest azure (Armenio) of the painter's palette, others the flame of burning sulphur, or of a fire quickened by oil." This charming iridescence is not however inherent in the stone, but produced by the reflection and refraction of light in certain openings in the interior of its mass, which are not fissures, but arranged in regular directions. It is composed of pure Silica and water, the latter imbibed from the atmosphere. This structure renders it extremely tender ; sometimes even flying into pieces on a sudden exposure to either extreme of cold or of heat. It is subject also to the loss of its colours, when the openings on which the-play of colours depends become choked up with dust and grease in wear ; although it is said they may be recovered by the hazard­ous operation of roasting the stone by a gradual heat. The Hungarian Opals exhibiting an uniform milkiness of surface, more or less iridescent, have from their greater density the advantage of resisting the effects of wear longer than any other sort ; hence their superior value. But infinitely greater is the beauty of the Mexican when recent, presenting an unmixed globule of green fire like the glowworm's lamp, or a ball of phosphorus moistened with oil. Nevertheless of so porous a nature is this kind, that it becomes colourless if wotted ; and changes to an opaque brown after a brief existence as a jewel, and consequently it has no value in the gem-market.