Crystal
Gypsum so soft you can scratch it with a fingernail, and so clear the Romans glazed windows with it.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Selenite is the clear, crystalline form of gypsum, and it is the softest stone in general circulation — hardness 2, which means a fingernail marks it and a damp cloth will dull it. It also dissolves in water. Every so often someone leaves a selenite wand in a bowl overnight to cleanse it and finds a chalky ghost in the morning; this is the one piece of practical advice in the whole compendium that is worth memorising.
It grows very large. The Cave of the Crystals at Naica in Mexico, opened by mine dewatering in 2000, holds gypsum beams over ten metres long — among the largest natural crystals ever found — grown over hundreds of thousands of years in water held just below the point at which anhydrite converts to gypsum. Romans mined a related material, lapis specularis, and split it into thin translucent sheets for windows before glass was cheap.
Practice uses it for clearing more than for anything else: passed over other stones, set in a room, laid in a grid. The reasoning offered is that it does not need clearing itself, which is convenient, unfalsifiable and very widely believed.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Laid across a shelf of other stones to clear them, on the understanding that selenite needs no clearing itself. Where this idea came from is genuinely obscure; it is not in any older source and appears to have spread through the trade in the 1980s.
Carried forward
Heinrich Cornelius Agrippa, Three Books of Occult Philosophy (1533)
Agrippa places the pale, translucent and silver-white stones among the Moon's, grouping them by appearance and by what he takes to be their sympathy with lunar moisture. The reasoning is analogical throughout — a stone that looks lunar is lunar — and he does not pretend otherwise.
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.