An amethyst stone is a type of gem which is soft and always violet in appearance. It costs 600 zorkmids. Each version of Mine's End contains an amethyst.
Dipping an amethyst in a potion of booze will turn it into a potion of fruit juice. This is also a good way to distinguish fruit juice from see invisible since they taste the same.
In slashem, an amethyst can be used in gem alchemy by dipping it in acid to produce a magenta potion.
The ancient Greeks believed that the stone protected its owner from drunkenness. The name comes from the Greek ἀ a- (“not”) and μέθυστος methustos (“drunk”). This is why it transforms booze into fruit juice in NetHack.