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Ch. 13: Olivine - Sphene

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	260	PRECIOUS STONES.

	absorption bands in the spectrum, as was indicated by Professor Church. The specific gravity is very variable, usually 4^-6 to 4'7, but it may range from 4.2 to 4^-86. The fracture is conchoidal, the mineral is brittle, and there are two cleavages, one prismatic and the other parallel to the faces of a pyramid, but neither are distinct. The hardness



	is 7-1/2 the streak is uncoloured. Zircon crystallises in the tetragonal system, and is usually found in rather elongated forms, as shown in Fig. 32. Some crystals are unequally developed and complex (Fig. 33); more rarely the prism faces are poorly developed or absent. Twinned forms occur> the component parts meeting one another in a form likened to the bent knee. Zircon is found as a secondary mineral

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