Precious Stones: Olivine - Sphene Page 272



	PRECIOUS STONES.	269

	In crystalline form it is orthorhombic, and the habit is usually prismatic, though in the variety Chiastolite it is more often acicular. In composition it is a silicate of aluminium, A1₂0₃ SiO₂ The variety Chiastolite shows a cross when cut in a plane normal to the vertical axis, and when in large crystals is sometimes cut as a charm. Like all strongly pleochroic minerals, the colour effect must be kept in mind in cutting. A good specimen when cut resembles Alexandrite, but is softer and less dense. 400. Cyanite. Cyanite, known also as Sappare and Kyanite, is closely related to Andalusite, both in origin and composition; and, like Andalusite, though fairly widely distributed, is but rarely found in a form suitable for gem use. It is found in various shades of blue, green, and grey. The crystals often show a good colour in the centre, but are pale or white at the margins. The lustre is generally vitreous, but is pearly on the cleavage surfaces ; transparent specimens are rare. The double refraction is rather strong, the mean index being given as 1.72. Pleochroism is well marked, and in the dichroscope images of two different depths of blue are seen. It is infusible ; the specific gravity is 3'58 to 3'68. The hardness shows an unusual variation in the different directions, the least being between 4 and 5, and the greatest as much as 7-1/4. It is a rather tough mineral. There are two cleavages, one perfect, the other less so. In crystalline form it is triclinic, and it usually occurs in embedded and elongated forms, with no definite termination; the crystals are often bent. Most of the gem varieties are supposed to have come from India, but the exact locality is