PRECIOUS STONES.	239
	spoken of as pleochroic. It is transparent to translucent, and has a vitreous lustre on the crystal faces. It is doubly refracting, the greatest index for yellow light being 1'543, and the least 1.537. The dispersion is feeble. It is only fusible at high temperatures, and it is but slightly attacked by acids. The specific gravity varies from 2-60 to 2-72, depending largely on the amount of iron present. It has a conchoidal fracture, and a hardness of 7 to 7-1/2. It has a distinct cleavage normal to the longer horizontal axis, and indistinct cleavages parallel to the other two axes. In crystalline form it is rhombic, but distinct crystals of any size are rare, and when they are found are usually rough. The general habit of the crystal is prismatic, and rather short. Twinned crystals sometimes give rise to a pseudo-hexagonal form {see Chrysoberyl). More often the mineral occurs in rolled fragments or massive. Occasionally com­pletely developed crystals are found in granite, as in Bavaria, but usually it is only found in association with rocks which have undergone great metamorphism. In chemical composition it is a silicate of aluminium, with magnesium and iron, and often with calcium, and containing water. The formula is possibly H2O4 (Mg, FeO), 4 A1203, 10 Si02. The colour is probably due to ferrous iron. The most commonly associated minerals are Quartz, Orthoclase or Albite, Tourmaline, Garnet, Andalusite, Beryl, Topaz, etc. It occurs in many parts of Scandinavia, as at Arendal and Kragero in Norway; at Abo in Finland ; at Tunaberg in Sweden; in Greenland, etc. But the chief gem qualities come from the gem gravels of Ceylon ; there stones of both light and dark blue are found, specimens of a sky blue being known as "Water Sapphire or Saphir d'Eau,