Loose Gemstones | Gem Art | Gem Stone Information



	XIV

	OPAL For variety and beauty of color the " precious" opal is without a peer. These colors do not come from any chemical constituent, but are produced by structural peculiarities which, in varying degrees, diffract the light rays entering it and give a prismatic play of colors. It is composed of silica with six to twelve per cent, water, and occurs in thin, irregular veins within a matrix. Although always a soft stone, its hardness is increased by exposure to the air. It is infusible before the blow-pipe, but heat drives off the water and renders it opaque. The combinations of color are infinite. The most beautiful opals are called " harlequin." These are distinguished by small, angular patches of brilliant and variegated colors, which change their positions and character as the stone is moved. Others show large blotches of color which change as the stone is turned, but seldom show more than two colors at the same time, one being much more pronounced than the other. Usually the colors of the opal are broken up into small speckled lights, which play as it is moved. The rarest form is the cat's-eye opal, which exhibits a chatoyant line over the centre of the dome similar to the cat's-eye and usually of a bright-green color. Formerly this stone was obtained almost exclusively from Hungary, the principal mines being in the Libanka Mountain, west of Dubnik, and not far from Czerwenitza. It is found there in the clefts and cavities of an old lava, generally brown in color, known as andesite. It is supposed that alkaline waters decomposed this rock and, setting free the silica, 112