Amethyst is a somewhat-valuable purple quartz crystal found everywhere as small clusters. It also has the distinction of being dropped by amethyst men upon death.
In real life
The most famous and precious quartz variety, colored by significant gamma radiation exposure. Originally named for its believed magical sobering qualities (Gr. amethystos, "one that does not get drunk"). This clearly does not work in Dwarf Fortress (or in real life). Historically it was also much rarer in the Old World until large deposits were discovered in Brazil. Before this it was ranked among the cardinal gems along with diamond, emerald, ruby, and sapphire.
[SOLID_DENSITY:2650] Common to quartz gems