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PRECIOUS STONES.

the sapphirine quartz, the jasper, etc; but except in the forms of cameos and intaglios these stones are but little esteemed. Rich deep-coloured amethysts, with the colour not quite uniformly distributed throughout their substance, are perhaps the most prized form of quartz. The colour of such amethysts gains in beauty from the dichroism of the stone and from its peculiar rippled and parquetry structure. Such amethysts are wrongly called oriental amethysts by jewellers (some do come from India), for the true oriental amethyst is a purple sapphire, not quartz at all, and an excessively rare stone.

The localities for choice specimens of amethyst, sard, chrysoprase, chalcedony, etc. are legion. The amethysts of Brazil and Ceylon, the agates of Uruguay, the chrysoprases of Silesia, the cornelians of Arabia, and the jaspers of Egypt are famous.

Not only have the dark-coloured onyxes of commerce been artificially dyed or stained, but a large proportion of agates, cornelians, sards, etc. have been similarly altered. A moderate heat reddens many varieties of quartz originally grey and brown, while a soaking in sugar or honey, followed by treatment with strong sulphuric acid, brings out black and white bands in the natural grey onyx.

Hydrochloric acid develops a lemon-yellow colour in white chalcedony, while a strong blue colour may be imparted by causing Prussian Blue to be precipitated within the stone by alternate soaking in solutions of green vitriol and of prussiate of potash.

Lapis-lazuli.

Lapis-lazuli or azure stone is not a definite mineral but a mixture, in variable proportions, of several minerals. Generally calcite forms the chief part of the colourless patches in the stone, but there will be present two or even three complex silicates possessing a beautiful blue colour. One of these is known as Haüyne or Haüynite, another is true ultramarine, and another is called sodalite. They are all silicates and all contain much alumina; but soda is also