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166
PRECIOUS STONES.
thin section across the bands acts as a diffraction grating when held to the light, and produces a spectrum.
The Agate is mentioned by both Theophrastus and Pliny, though " Achates" included many other substances besides this. They were used very largely by the Greeks for intagli, and by the Romans for their supposed medicinal virtues.
Although Agates, as known cut, are of such various colours, many of these colours are due to natural changes, after formation, and still more to unnatural staining by the cutter of the Agate. As already stated, Chalcedony is either colourless or is tinted with grey, which may, in thick pieces, appear a slate-grey to blue-grey ; this gives the predominant tint to an unweathered Agate. The presence of disseminated zeolitic material in those parts known as Cachalong bands produces various soft tints of cream and lavender colour. The outermost part of all is coloured by Saponite, to a dull green, and the whole Agate may have its Chalcedony so mixed with Saponite or Celadonite that its colour may be deep green. More rarely an iron compound (probably in the ferrous condition) is carried in, and on oxida­tion yields either the red oxide of iron Haematite, or the yellow hydrate Limonite, with the formation of bands of Jasper of these colours. Where the ferric oxide has separated as minute spheroids, the colour is a transparent one instead of opaque, and then the Agate is termed Carnelian Agate. Other Agates show a later infiltration by iron salts with the deposition of Haematite in certain bands only, for once formed, some bands hardly absorb any such solution at all; such specimens have quite a distinct appearance from the Carnelian Agates. Another later change which may affect