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).
[SOLID_DENSITY:2650] Common to quartz gems