Crystal
Opaque, patterned, everywhere — the stone that gets picked up off a beach and kept for forty years.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Jasper is chalcedony carrying so much foreign material — iron oxides, clay, volcanic ash — that it is completely opaque. Where agate is translucent and banded, jasper is solid and patterned, and the patterns come from whatever was suspended in the silica as it set: brecciated fragments, dendrites, orbicular rings, landscape scenes that people name and sell accordingly. It is one of the commonest ornamental stones on Earth and turns up in beach gravel on most continents.
The name descends through Greek iaspis from a Semitic root, and the ancient iaspis was probably green and probably not what we now call jasper — quite possibly chrysoprase or a green chalcedony. This matters because jasper appears in scriptural lists of stones, including the breastplate in Exodus and the walls of the New Jerusalem in Revelation, and those identifications are much less secure than the confident lists in old lapidaries imply. Where a translation says jasper, the honest reading is a green stone of some kind.
Practice keeps it as the plain, hard-wearing stone: endurance, patience, presence. It is also the stone people most often own without having bought — the one from a river bed or a holiday beach — and that provenance seems to matter to practitioners considerably more than any correspondence assigned to it.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Kept because of where it was found rather than what it is. The stone picked up on a particular beach on a particular day carries, for most practitioners who own one, a good deal more weight than any list of correspondences.
Carried forward
Pliny the Elder, Natural History, Book 37 (c. 77 CE)
Pliny writes that iaspis is green and often translucent, that it is worn throughout the East as an amulet, and that many peoples value it above all other stones. Nothing in that description matches the opaque red material sold as jasper today, which is a useful caution about reading old lapidaries as catalogues.
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.