A form of Quartz, it has only relatively recently been discovered that the colour is a result of certain iron compounds exposed to radiation in the rocks in which it formed.
In fact all coloured Quartz gems can be susceptible to quite modest temperature changes and radiation. So best not to take your Amethyst jewellery into places that have a lot of UV light (such as the local disco or tanning salon).
The Greek word 'amethystos' can be translated as 'not drunk' and the ancients considered it to be a strong antidote against drunkenness, which is why wine goblets were often carved from it. Legend has it that when a drunken Dionysus was pursuing a maiden called Amethystos, who refused his affections, she prayed to the gods to remain chaste. The goddess Artemis granted the prayer, transforming her into a white stone; humbled by Amethystos' desire to remain chaste, Dionysus poured wine over the stone she had become as an offering, dyeing the crystals purple.