AMETHYST, properly, is only a variety of quartz or rock-crystal distinguished by its fine violet-blue or purple colour. This tint seems to be caused by a minute mixture of the peroxide either of iron or of manganese, and is lost when the stone is exposed to the action of the fire. It then changes through yellow and green to colourless; and in this condition is often sold for the aquamarine or topaz. Amethyst is generally found in thick columnar masses, of short hexagonal prisms termmating in pyramids. The faces, especially in Brazilian specimens, are often marked by zig-zag or undulating lines, and the colour in many is similarly disposed, showing a peculiar internal structure in the stone. It has been proposed to name all varieties of quartz, whether coloured or uncoloured, showing this peculiarity, amethyst, but without sufficient reason. Amethyst, according to Plmy, got its name, άμέθνστος, from its supposed power of preventing drunkenness. Though not a true gem, it was formerly much valued as an ornamental stone, but has greatly declined in value in the present century, being obtained in great abundance from Brazil. There it is often white or yellow, and named topaz. The finest blue stones are found in Ceylon and Siberia; and less remarkable ones in many places in Europe, India, and Australia. Amethysts may be counterfeited by glasses, to which the proper colour or stain is given by mineral matter. There were fine ones made in France about the year 1690, which even imposed on connoisseurs, but with the decrease in price there is now less danger of such deceptions.
9th ed. Encyclopædia Britannica, vol. 1, p. 736
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