Crystal
Purple quartz, once as costly as ruby — and the stone the Greeks hoped would keep them sober.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
The colour comes from iron in the quartz lattice reworked by natural radiation over geological time, which is why heating amethyst turns it yellow: you are undoing the irradiation. Almost all commercial citrine is amethyst that has been through an oven, and the two stones are therefore the same material at different points on a single curve — a fact that makes their opposed correspondences slightly awkward to defend.
Until Brazilian and Uruguayan deposits opened in the early nineteenth century, amethyst was a genuinely scarce stone and priced with the great gems; the huge geodes that make it a five-pound purchase today are a nineteenth-century development. The Greek name amethystos means roughly not-drunken, and the claim attached to it — that a cup of amethyst, or a stone worn while drinking, kept a person clear-headed — is one of the oldest specific stone claims on record. It is also one of the oldest to be publicly doubted; Pliny reports it and does not think much of it.
Modern practice moves it upward, to the crown and the third eye, and towards sleep and dreaming. That is a substantial shift from sobriety, and the throughline offered is usually something like clarity — which is broad enough to accommodate almost anything.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Pliny the Elder, Natural History, Book 37 (c. 77 CE)
Pliny records that the magi promised amethyst would prevent drunkenness, and that engraved with the names of the sun and moon it would win access to princes. He calls these promises falsehoods, and adds drily that the name most likely comes from the stone's colour approaching wine without quite reaching it.
Carried forward
Common in contemporary practice
Kept by the bed for sleep and for remembering dreams. This is a twentieth-century placement with no classical warrant, and it sits oddly beside the old sobriety lore, but it is now much the commonest use.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.