Crystal
Fossil driftwood from a Jurassic sea — light as a feather, black as anything, and the whole of Victorian mourning.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Jet is not a mineral either. It is wood — most often monkey-puzzle, driftwood that sank into anoxic mud on a Jurassic seabed and was compressed over a hundred and eighty million years into a hard, close-grained coal that takes a glassy polish. It is astonishingly light. Anyone who has handled jet and cannonel side by side knows the difference at once, and the lightness is the honest test against the black glass and plastic sold in its place.
Whitby on the North Yorkshire coast has been the great source since the Bronze Age, and the town's jet industry peaked spectacularly in the nineteenth century: when Victoria went into mourning for Albert in 1861, jet was one of very few ornaments the etiquette of full mourning permitted, and at its height Whitby had some two hundred workshops. That is a real, dated, documented episode of European stone lore, and there are not many of those.
Rubbed hard, jet takes a static charge and picks up small pieces of paper — the same effect amber has, and one reason the two stones were paired in older lapidaries long before anyone had a word for electricity. Practice keeps that pairing: jet for the dark side of protection and grief, amber for the warm.
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 names gagates for a town and river in Lycia, notes that it is black, light and not very hard, and records the odd fact that it takes fire from water and is quenched by oil. He is describing a bituminous coal, and reporting a laboratory observation of the sort that makes his stone book worth reading.
Carried forward
Common in contemporary practice
Worn in grief, following the Victorian custom rather than any older one. Practitioners tend to describe jet as a stone for carrying something rather than for shedding it, which is a distinction the mourning-jewellery tradition arrived at on its own.
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.