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154
PRECIOUS STONES.
downward percolating water. The formation of one pound of Amethyst in a vapour cavity has been estimated to occupy over one and a quarter million of years (Rudler, " Pop. Science Review," Vol. I.).
What the colour is due to is not absolutely certain, but probably most specimens owe it to the presence of traces of manganese compounds, which impart an almost identical tint to glass when fused with it. Manganese is certainly present in many Amethysts, though in very small percentages, a deep coloured stone only showing O'Ol per cent. (Heintz). The colour is easily discharged by heat.
It is found in many localities. In England, chiefly in the south-western counties—Cornwall, Devon, Somerset and Gloucester. In Scotland, at many places, as Loch Morar in Inverness-shire, Campbeltown in Argyllshire, etc. Ireland, near Cork, and on Achill Island, both providing fine speci­mens. Abroad, at Oberstein, there was at one time a plentiful supply, but now it is exhausted, though the chief seat of the cutting industry is still there, the material being imported from Brazil, where it is found in large amount. Thus in the Serra do Mar a cavity was found containing 35 tons of Amethyst in 1900 (Max Bauer). Uruguay also provides a good deal. At Mursinka in the Urals it is largely worked, here occurring in veins and cavities in decomposed granite, other cavities at greater depths con­taining Beryl and Topaz. Transylvania also yields a certain amount. In China large crystals are found, but most of them are turbid. Ceylon yields the finest qualities, it being here found in gravels derived from the weathering of granite rocks, associated with many other gem stones. In North