Quality and Sources
A good amethyst should be of a deep purple colour, perfectly transparent and throughout uniform in hue. Amethysts are distinctly dichroic; they rank No. 7 in the Mohs scale of hardness; specific gravity is 2.6 to 2.7. The crystallisation of this quartz is in six-sided prisms terminating in pyramids. Lustre vitreous; cleavage none or distinct; fracture conchoidal, glassy. It is doubly refractive, the twin colours being reddish and bluish purple. Amethysts are usually cut step, while the finer specimens are cut brilliant.
The chief sources of supply for amethysts are Brazil and the Ural Mountains, Siberia. The Siberian amethysts, accompanied by beryl and topaz, occur in cavities in granite; often they are found lying loose and sometimes very near the surface. Cavities in a black eruptive rock (melaphyre) are the hiding places of some Brazilian amethysts, while others are found as pebbles in the river gravels with chrysoberyl and topaz as companion minerals. Gem amethysts are also found in gravel bearing other gems in Ceylon.
In North America, a few of the finest specimens of amethyst on record have been found in Oxford County, Maine. Other localities are Delaware and Chester Counties, Pennsylvania, and Haywood County, North Carolina. Crystallised amethyst in commercial quantities has been found at Thunder Bay on the north shore of Lake Superior. The crystals are highly coloured but not uniform or clear and few good gems have been obtained there.
Amethyst was formerly much more highly prized than now because of its scarcity. Besides the increased supply it has been imitated so convincingly as to impose upon all excepting gem experts. A celebrated amethyst necklace owned by Queen Charlotte of England, valued at $10,000, might not now be worth intrinsically $500. The exclusive charming violet colour of the amethyst will probably always insure a demand for the best qualities of this stone, and with a development of art in the treatment and uses of precious stones and jewelry, the demand is likely to grow.
Of all specimens of amethyst that appear in the market to-day, the Siberian stones so far outclass all competitors in richness and depth of their dark violet hue, that these beautiful gems mixed with others would be instantly selected by the merest novice; so manifestly superior is their quality that, comparatively speaking, they alone are gems, and the only reason that their cost is not much greater than it is, is because Nature has been generous in the quantity that she has permitted man to extract from her mineral treasure house.