116 A Book of Precious Stones 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 condi-tion 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. Le-chosos 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 cqntains iron, and is to opal as jasper is to quartz. Wood opal is wood silicified by opal. Hyalite (Miiller's glass) is colourless and clear, or trans­lucent and a bluish white. Moss opal contains