Crystal
Chalcedony in straight parallel bands — the cameo carver's stone, and mostly dyed when it is black.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Properly, onyx is chalcedony with straight parallel bands, as against agate's curved and concentric ones. When the bands are white against red-brown it is sardonyx, and that is the material Greek and Roman carvers used for cameos: cut a layered stone at the right depth and you get a pale figure standing out from a dark ground, with no colouring involved at all. The technique demands both patience and a good eye for what lies under the surface, and the surviving Roman cameos are among the most technically astonishing small objects of the ancient world.
Almost all solid black onyx on the market has been dyed. The process — soaking porous grey chalcedony in a sugar solution and then in acid, which chars the sugar to carbon inside the pores — was known by the nineteenth century and is entirely standard. It is not fraud, exactly, but very few sellers volunteer it.
Practice puts onyx with Saturn and with the disciplines: boundaries, endings, keeping a secret. Its older reputation was frankly bad — several classical and mediaeval sources treat it as a stone that brings discord and troubled sleep — and the modern rehabilitation into a protective stone is a genuine reversal, not a continuation.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Heinrich Cornelius Agrippa, Three Books of Occult Philosophy (1533)
Agrippa gathers the black, heavy and earthbound stones under Saturn, alongside lead and the colder herbs, on the reasoning that like belongs with like throughout the chain of correspondences. Onyx sits in that company, and its long association with limits and endings descends from this placement rather than the reverse.
Carried forward
Common in contemporary practice
Worn to close something off — the end of a period of mourning, the end of a contract, a boundary being set. It is one of the few stones in general use whose reputation was frankly unfavourable for most of its history and has been deliberately turned round.
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.