Amethyst is sometimes heat treated to intensify the color.
Fake Amethyst and How to avoid it
There is not much risk of buying fraudulent amethyst. Since amethyst is not especially rare, nor expensive, there is not much incentive to sell fakes. However, possible imitations could include purple glass or in rare cases, synthetic purple corundum. The main difference between amethyst and glass is that glass is much softer and can scratch easily. Corundum will have a much more reflective (mirror-like surface) and will not scratch as easily as amethyst. If you look inside the stone and you notice any color bands, that is a sure-fire way to identify amethyst.
Another sign of fake amethyst is its price compared to clarity and color. If you see piece of amethyst jewelry in which the amethysts are a deep rich purple and perfectly clear, while the price of the piece is under $20, you have reason to suspect that it’s fake.
For example, these “amethyst” earrings are for sale on ebay for 99 cents: they are probably not real.