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72
PRECIOUS STONES.
 
 

 
 
and callainite and known as lazulite. It is essentially a hydrated phosphate, but contains besides aluminium, a considerable pro­portion of magnesium and iron ; copper is generally absent. It is softer and rather heavier than turquoise, and is far more difficult to dissolve in acids than that species. In colour it varies from a fine sky-blue to a tint not far removed from that of pale lapis-lazuli—a perfectly distinct mineral. Lazulite was occasionally used in ancient times as an inlay, for instance in the gold armlet from the Oxus now preserved in the British Museum.
Topaz.
Although the topaz is a perfectly definite and distinct mineral species, yet three different stones are commonly called by this name. But the topaz known as " oriental topaz " is in reality the yellow sapphire, a kind of corundum ; the occidental or Scotch topaz is nothing but yellow quartz ; while the true topaz, some­times spoken of as the Brazilian topaz, is the only one which in reality may properly bear the name. The hardness and specific gravity of these three stones are very different, and furnish good criteria for their discrimination :
The true topaz belongs to the orthorhornbic system ; its crystals are prisms, usually having but one end regularly terminated. The cleavage of topaz is highly perfect and basal, that is, trans­verse to the length of the prism. The prismatic faces are commonly deeply channelled but brilliant. The refractive indices of topaz, in the three directions of the axes, are, for the yellow ray in a colourless crystal :
The double refraction of topaz is strong, and the pleochroism of coloured specimens very marked. A wine-yellow crystal from