Crystal
Schorl — a ridged black column that really does develop a charge when you warm it.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Schorl is the iron-rich end of the tourmaline family and by far its commonest member; the striated black prisms that turn up in granite pegmatites the world over are the same species as the pink and green gem tourmalines, differing only in what metal sat in the melt. The name has been in German mining vocabulary since at least the fifteenth century, long before the mineral had a chemistry.
Tourmaline is pyroelectric and piezoelectric — heat it or squeeze it and opposite charges gather at the ends of the crystal. Dutch traders in the eighteenth century used warmed tourmalines to pull ash out of their pipes and called the stone aschentrekker, ash-drawer, which is a genuinely lovely bit of practical folk physics. This is also the most abused fact in crystal writing: the effect is real, it is tiny, and it requires a temperature change. A tourmaline on a bedside table is not doing it.
What practice makes of it is protection of the shielding kind — placed at thresholds, carried in a pocket on a hard day, set at the corners of a room. Of all the black stones it is the one most often described as turning things away rather than taking them in, and the pyroelectric story is doing a lot of work in that reputation whether or not it should be.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Set at the four corners of a room or either side of a door. The placement is borrowed from older threshold customs involving iron and salt, and the stone has largely taken their place in households that would rather not hammer nails into the lintel.
Carried forward
Common in contemporary practice
Frequently invoked as absorbing electromagnetic radiation from screens and routers. There is no evidence for this and the underlying physics does not support it; it is included here because the claim is common enough that a reader will meet it, not because it stands up.
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.