Onyx is a stone. If you cut it, you see lines — bands of colour sitting in layers. The kind people know best looks almost black, often with a thin white line, and they cut it into rings, beads, and little boxes. Though actually, it is not only black: it can be brown, white, or mixed, and shops sometimes use the name a bit loosely for any banded stone that looks like it. You meet it in jewellery shops and in talk about old objects, not in everyday chat. It is a real stone name, not a brand. The old Greek word first meant a fingernail; the layers in the stone looked a little like the layers of a nail, so they reused the name.