Amethyst is the violet variety of quartz, silicon dioxide, and its colour has an unusual cause: it requires both iron impurities and natural irradiation from surrounding rock over geological time to produce the colour centres responsible. Heat destroys them, which is why amethyst fades in strong sunlight and why heating amethyst produces citrine, a very large proportion of commercial citrine is heat-treated amethyst, generally undisclosed.
The name is Greek, amethystos, and it means not drunken. This etymology is secure and is one of the few gem etymologies that is not disputed. The belief attached to it, that the stone prevented or delayed intoxication, is attested in antiquity, and drinking vessels of amethyst and amethyst-coloured glass were made. Whether the belief generated the name or the colour did is less clear: wine-coloured water in a cup looks like wine and is not, which is at least as good an explanation of the association as any pharmacological claim.
The romantic myth usually printed, the nymph Amethystos turned to stone to escape Dionysus, the god pouring wine over her in remorse, is not ancient. It was written by the French Renaissance poet Remy Belleau in the sixteenth century, in a collection of poems about gems, and it is his invention. It appears in gem books as Greek myth constantly. This is a checkable point: the story is absent from every classical source and its author is known.
Amethyst was a costly stone until large Brazilian and Uruguayan deposits were opened in the nineteenth century, after which it became inexpensive. Its status in older sources reflects the earlier scarcity.