Every version of Mines' End contains an amethyst.
Dipping an amethyst in a potion of booze will turn it into a potion of fruit juice. This is a good way to identify both the potions of booze and fruit juice as well as the amethyst, since no other gem transforms a potion by being dipped into it. Identifying fruit juice will also distinguish it from the potion of see invisible, which has the same messages.
The amethyst is a real-life gemstone; the ancient Greeks believed that the stone protected its owner from drunkenness. Its name comes from the Greek ἀ a- (“not”) and μέθυστος methustos (“drunk”), which is why it transforms booze into fruit juice in NetHack.