Nature and Varieties
The precious opal is one of the most individual of gems; of all the opaque minerals, it reveals the most beautiful play of colours, in folklore it is the birth-stone of October and the symbol of hope, and yet, for years, the fame of this fire-flashing stone was blackened by a cloud of superstition which condemned it as unlucky; a superstition the origin of which is obscure. For a time, however, it largely regained its lost popularity, having found its most illustrious patron in Her Majesty, the late Queen Victoria. Another remarkable fact about the opal is that it is not found in the Orient—the very land of gems.
Opal, in mineralogy, is Hyalus opalinus, of the order Hyalina; it is of granular structure; small reniform and stalactitic shapes and large tuberose-like concretions; hardness 5.5 to 6.5; specific gravity 2 to 2.21; lustre vitreous, sometimes inclining to resinous or pearly; streak, white; colour, white, yellow, red, brown, green, or gray. The colour is usually pale, due to foreign elements. Some opals exhibit a rich play of colours, while others present different colours by refracted and reflected light. The cause of the colour-play is the physical condition resulting from a multitude of fissures having striated sides which diffract and decompose the light. The chemical composition of the opal is ninety per cent. silica and ten per cent. water.
Besides precious opal, there is the harlequin opal which presents a variegated play of colours on a reddish ground, and resembles the fire opal which shows hyacinth red to honey-yellow colours, with fire-like reflections. Girasol is bluish-white and translucent, and, under a strong light, presents reddish reflections. Lechosos opal is a variety remarkable for flashes of green. Hydrophane, a light coloured opaque kind, becomes transparent when immersed in water. Cacholong is an opaque porcelain, bluish, yellowish, or reddish white. Opal agate has an agate-like structure. Jasp opal contains iron, and is to opal as jasper is to quartz. Wood opal is wood silicified by opal. Hyalite (Muller's glass) is colourless and clear, or translucent and a bluish white. Moss opal contains manganese oxide, and is to opal as moss agate is to quartz. A freakish variety of opal is tabasheer, a silica deposited within the joints of bamboo; it is absorbent, and, like hydrophane, becomes transparent when immersed in water.
As a mineral, opal is quite common, so that an amateur's collection of minerals can include specimens to represent opal—some of them very beautiful, too—at small cost, or for the effort of prospecting, in many localities. The varieties of opal are many, and the frequent inclusion of foreign matter invests it with a wonderful variety of colours. The silica deposited by nearly all natural hot waters is opalescent. The Yellowstone Park geysers shoot up around cones of opal raised by the constant accretions of silica deposited by the passing hot waters, which fall into opal basins created in the same way. This variety of opal is termed geyserite. There is a wide gulf in values between precious or noble opal—the gem stone quality—and opal in general.
Opal is generally found filling seams, cavities, and fissures in igneous rocks, also embedded in limestone and argillaceous beds.