Page:Handbook of Precious Stones.djvu/121

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LABRADORITE.
105

Labradorite

Labrador spar is a felspar, crystallising in the triclinic system. It is usually translucent rather than transparent, and by transmitted light appears of a grey colour. Owing chiefly to a peculiarity in its intimate or minute structure, which resembles a complex system of gratings, labradorite often shows magnificent chatoyant reflections of brilliant blues, sea green, orange, puce, amber, and peach-blossom hues, in fact, a coloured metallic sheen. It should be cut with a nearly plane or but slightly convex surface. It occurs, associated with hypersthene and amphibole, of fine quality in the island of St. Paul, and in large masses on the coast of Labrador; also in Finland, Volhynia, the United States and Queensland. Labradorite has the hardness 6, and the specific gravity 2·7 to 2·75. In 100 parts it contains:

Silica 55·5

Alumina 26·5

Iron oxides 2·0

Lime 11·0

Soda 4.0

In some specimens there is less lime, but instead, a small percentage of potash and magnesia.


Moonstone or Adularia

This is a variety of felspar, or rather of that species of monoclinic felspar called orthoclase or orthose. Moonstone is found at St. Gothard, and very abundantly in Ceylon. It differs from ordinary orthoclase in the remarkable pearly reflection of light which it exhibits in certain directions. The most esteemed specimens are those in which the chatoyancy has a distinctly bluish hue. Some varieties are nearly opaque; a chocolate-coloured sort has also been found. The bluish stones only possess a market-value. The hardness of moonstone is 6, and its specific gravity almost invariably 2·58. It contains in 100 parts:

Silica 64·5

Potash 15·0

Alumina 18·5

Soda 1·0

with traces of lime, and magnesia.