Up until the 18th century, amethyst was included in the cardinal, or most valuable, gemstones (along with diamond, sapphire, ruby, and emerald). However, since the discovery of extensive deposits in locations such as Brazil, it has lost most of its value. Collectors look for depth of colour, possibly with red flashes if cut conventionally.
As amethyst is readily available in large structures the value of the gem is not primarily defined by carat weight; this is different from most gemstones where the carat weight exponentially increases the value of the stone. The biggest factor in the value of amethyst is the colour displayed.
The highest-grade amethyst (called “Deep Russian”) is exceptionally rare and therefore, when one is found, its value is dependent on the demand of collectors. It is, however, still orders of magnitude cheaper than the highest-grade sapphires or rubies