Amethyst is, by definition, the purple variety of quartz. For this reason there is no such thing as green amethyst, and the stone thus labelled is simply green quartz. Colours range from almost pink to a rich deep purple. Quite often the colour is artifically enhanced by irradiation, which is virtually impossible to detect. As the end result is pleasing, most people have no objection to this, the question simply remains as to value. World supplies are plentiful, and even if all the naturally coloured ran out, there is an almost endless supply of quartz that can be irradiated.
Therefore even the highest quality amethyst is a good, economical alternative to more expensive precious gemstones and is increasingly popular in mainstream jewelry, despite generally being outrageously over-priced. Interesting to note is that the banded (striped) stone, which was often seen as poorer quality in previous decades, is now enjoying favour much as it did in Roman times.
Another quartz, the yellow citrine, is generally heat-treated amethyst. Natural citrine is extremely rare and extremely expensive, not often found on the normal market. A related stone, ametrine, a dual coloured combination of amethyst and citrine, also occurs naturally, but in very limited quantities, and irradiated quartz is again the most widely available stone. Customers should not be afraid of artificially enhanced gemstones, but should consider value accordingly. As the mark-up on all gemstones is high, it might be fair to take into account the expense incurred in these enhancing processes. In any case, a stone is worth whatever the customer is willing to pay for it.
Amethyst and its close cousins are relatively easy to shape, and therefore many different beads and pendants are sold, in addition to the cut stones. Natural crystals are also available in considerable size, and there are whole caves to visit if you are a dedicated fan.