Crystalline quartz in colors ranging from pale lilac to deep reddish purple and ranging from transparent to translucent is known as amethyst. The wide range of colors and uses is shown below:
Siberian mines once produced the world's finest stones with particularly rich purple color that glowed with reddish and/or bluish highlights. Today the term "Siberian" is no longer is a place designation, as the mines are long since worked out, but instead is used a a "grade" term, implying colors similar to the original stones from Siberia.
The ready availability of this traditional February birthstone is a rather new development, as through most of history this gem was rare and highly valued especially by royalty and Church officials. In more ancient times this stone was believed to protect the wearer from the effects of alcohol, and to have a generally elevating effect on one's mental capacity.
Major sources are Brazil, Bolivia and Uruguay in South America and Zambia in Africa. Brazilian stones can be found in huge sizes, but generally are moderate in color and often suffer from color-banding, which sometimes is visible despite efforts of the cutter to minimize it. Many amethyst lovers prefer the usually smaller, but more richly colored stones coming from Zambia, and more recently from Uruguay. Most amethyst in nature occurs as hydrothermally precipitated crystals in cavities within volcanic or sedimentary rocks. The geode below is a Brazilian piece weighing 10.5 lbs and shows both the crystalline growth pattern and the natural color zoning typical of material from this source.
Very light amethyst, which once was considered low grade, has gained a recent boost in popularity by intensive exposure on TV shopping programs and the clever marketing strategy of calling it "Rose d' France". To my mind these light stones have their greatest appeal when given fancy and unusual cuts, where the artistry of cutting is more on display than the material itself.
At hardness 7 and with no particular warnings on care necessary, amethyst makes a fine jewelry gem for virtually all purposes short of engagement rings. Lower grades of material are cabbed, carved, and made into a great variety of beads and other ornamental objects.
Amethyst is commonly simulated by glass, and synthetic spinel doublets, both of which are easy for the trained eye to detect. Synthetic amethyst has been produced for decades, and until about 20 years ago was detectable with simple gemological tests. Natural amethyst crystals are "twinned" which creates a particular visual pattern when viewed under the polariscope -- the earlier synthetics were not. Most of today's synthetic quartz is produced in Japan and Russia.
Unfortunately as such things go, those who wish to deceive are always seeking ways to pass off synthetic as natural. The newer process involving growing the synthetic material on a natural seed crystal, resulting in twinned synthetics makes detection all but impossible. Much of the "natural" amethyst being sold in today's market is actually synthetic. Your best bet if you want amethyst from Mother Nature, and not from a lab is to either purchase natural crystalline rough and have it cut, or to get your cut gems or jewelry from a trustworthy source.
Value per carat in amethyst, unlike many gems, doesn't rise exponentially with weight, as it is readily available in large sizes, so its worth depends almost entirely on color. The "Siberian" deep purple with red and blue flash commands the highest prices. Brazilian stones generally look richer in color the larger they are, whereas the deeper Uruguayan and Zambian material often becomes too dark in larger sizes. Amethyst is plentiful in the marketplace, so there is no reason to pay top dollar for stones with visible inclusions or inferior cutting.