Crystal
Volcanic glass with an edge sharper than a scalpel — a stone that has never been gentle and does not pretend to be.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Strictly speaking obsidian is not a mineral at all. It is glass: rhyolitic lava cooled so fast that no crystals had time to form, which is why it has no internal structure and breaks in smooth conchoidal curves. Those curves are the point. A freshly struck obsidian edge can be a few molecules thick — sharper than surgical steel, and brittle enough to be useless for most things and unmatched for a few.
That edge made obsidian one of the first materials people traded over long distances. Blades struck from the flows at Göllü Dağ in central Anatolia travelled hundreds of miles in the Neolithic, and because each volcanic source has a distinct chemical fingerprint, archaeologists can trace those routes with real confidence. It was also worked extensively in Mesoamerica, where it was a principal material for cutting tools and mirrors.
Practice treats it as the uncompromising stone — used for cutting away rather than for comfort, and frequently described as showing you things you would rather not look at. The mirror lore is old and geographically wide, and polished obsidian really does make an excellent dark scrying glass, being black, hard and capable of taking an almost perfect surface.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Pliny the Elder, Natural History, Book 36 (c. 77 CE)
Pliny gives the stone a discoverer, one Obsius, who is said to have found it in Ethiopia — a very dark glass, he writes, sometimes used for mirrors that return a shadow rather than an image. The etymology is probably a mangling and Obsius is probably nobody, but the observation about mirrors is exact.
Carried forward
Common in contemporary practice
Used as a scrying surface, polished flat rather than domed. Practitioners describe the black mirror as showing nothing at all for a long time, and treat the tedium as part of the method rather than a failure of it.
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.