Crystal
Iron oxide with a mirror polish and a blood-red streak — heavy in the hand, and named for it.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Draw a polished hematite across unglazed porcelain and it leaves a rust-red line. That streak is the whole story of the name: the Greeks called it haimatitēs, blood-stone, and the association between this cold grey metallic mineral and blood is not mystical in origin at all — it is an observation any lapidary makes in the first week. Ground fine, hematite is red ochre, which puts it in the oldest human pigment record we have, on cave walls and in burials tens of thousands of years old.
Industrially it is the world's principal iron ore, and specimens run from the mirror-bright kidney ore of Cumbria to the specular schist of the Great Lakes ranges. It is dense enough that the weight is the first thing anyone notices, and weight is most of what practice makes of it: this is the archetypal grounding stone, invoked for the body rather than the mind.
The magnetic hematite sold on market stalls in bracelets is not hematite. It is a manufactured ferrite ceramic, sometimes labelled hematine, and while there is nothing wrong with it as an object it is not a stone that came out of the ground.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Carried for its weight rather than its colour. A polished palm-stone of hematite is noticeably heavier than it looks, and practitioners regularly describe the surprise of that weight as the working part — the stone insisting on the body.
Carried forward
Pliny the Elder, Natural History, Book 36 (c. 77 CE)
Pliny sets out several grades of haematites and connects the stone to Mars and to iron, which in his scheme are the same argument twice. He is following an association that the red streak makes almost inevitable, and the association has outlasted every framework built on 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.