Page:Handbook of Precious Stones.djvu/91

 ancients was our chrysolite and peridot, not the stone now called topaz, which was not known as a distinct stone until comparatively modern times. The topaz (pitdâh) of Aaron's breastplate was probably a peridot.

The commercial value of the topaz is small and variable. Very richly-coloured specimen stones, suitable for pendants may be bought for a pound or a few pounds; they are often sold by the ounce, not by the carat.

The tourmaline is marked out from all other precious stones by a very complex chemical constitution, and by a very interesting optical structure. Its hardness, 7·3 to 7·5, suffices to protect it from wear, while the range and quality of the colours which it exhibits commend it to those persons who appreciate the artistic value of jewellery in which other stones besides those which are well known and popular form dominant elements.

All the minerals called by the names "indicolite" (blue), "rubellite" (red), "schorl" (black), and "achroite" (colourless) form but one species—tourmaline. These differences of colour are accompanied by differences of composition, so that we have a series of varieties of tourmaline, in which, while the proportion of silica is fairly constant, the bases consist of the oxides of iron, magnesium, sodium, manganese, and aluminium in differing proportions. Water is also present, and sometimes lithia and potash. To give some notion of the chemical complexity of the tourmaline we may cite an analysis by Rammelsberg of a green Brazilian stone of specific gravity 3·107:

In 100 parts.

Silica 38·55

Alumina 38·40

Manganic oxide ·81

Ferric 5·13

Ferrous 2·00

Magnesia ·73

In 100 parts.

Lime 1·14

Soda 2·37

Lithia 1·20

Potash ·35

Boron trioxide 7·21

Fluorine 2·09