Which mineral produces a violet variety called amethyst?

Quartz produces the violet variety called amethyst.

Amethyst is crystalline quartz colored by trace amounts of iron and irradiation, with the exact color also influenced by defects in the crystal structure. Its shades range from pale lilac to deep purple.

Much natural amethyst forms in cavities of volcanic rocks, especially basalt. Silica-bearing fluids enter the cavities and deposit quartz crystals over time; later geological conditions help produce the purple coloration.

Amethyst is the birthstone associated with February and has a long history in jewelry and symbolism. Heating can change some amethyst to yellow or orange quartz, often sold as citrine, although naturally occurring citrine is also known.