Page:Handbook of Precious Stones.djvu/120

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104
PRECIOUS STONES.

pleochroism. Its crystals belong to the rhombic system. Good specimens, such as are occasionally met with in Ceylon, show in different directions of the crystal a soft lavender-blue, a greyish white, and a straw colour.[1] Iolite is frequently full of flaws and almost opaque; its beautiful change of colour is then very imperfectly seen. The hardness of iolite is above 7; its specific gravity is 2·6 to 2·66. One hundred parts of iolite on an average contain about:

Silica 49

Alumina 33

Ferrous oxide 5

Magnesia 10

Lime 1

Water 1


Crocidolite

Is, or rather gives rise to, one of the minerals which has been termed cat's-eye. It occurs of three distinct colours brownish yellow or gold (tiger eye), indigo or greenish blue, and dull red. When cut en cabochon of an oval form, with a high ridge, and with the longer diameter of the oval at right angles to the direction of the fibres or filaments which the mineral includes, crocidolite shows a good line of light and presents a brilliant appearance. It always contains a chalcedonic base; indeed the best specimens, which now come from Griqualand West, South Africa, and have a hardness of nearly 7, and specific gravity of 2·8, are essentially pseudomorphs after crocidolite, and not the unchanged mineral itself, which is softer and heavier. This stone is related to hornblende and asbestos, and has approximately this composition in 100 parts:

Silica 51

Oxides of iron 34

Soda 7

Magnesia 2

Water 3

Bronzite and Hypersthene are two other minerals, resembling crocidolite in their metallic reflections, and consisting of silica, iron oxides, and magnesia.


  1. Frontispiece, Fig. 5.