Crystal
The plain one. Silicon dioxide, water-clear, and the stone every other stone is measured against.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Quartz is the second most abundant mineral in the Earth's crust and the closest thing the craft has to a default. It is silicon dioxide, nothing else, and when it grows slowly in an open cavity it comes out in six-sided points so regular that the Greeks concluded it was water frozen past thawing — krystallos meant ice first and quartz second, and the confusion is still buried in the English word crystal.
Its reputation as the all-purpose stone is genuinely old, but the modern version of it owes a great deal to the twentieth century, when quartz turned out to be piezoelectric and went into every radio, watch and computer clock on Earth. That is real physics and it is worth being precise about what it means: a quartz plate under mechanical stress produces a measurable voltage and oscillates at a very stable frequency. It does not follow that a quartz point on a windowsill is doing anything. Practitioners who invoke the watch are reaching for a metaphor, and the honest ones say so.
In practice clear quartz is used as an amplifier and a blank — the stone you programme with an intent, or set beside another stone to lend it volume. That flexibility is exactly what makes it hard to say anything specific about.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Used as a blank: set beside another stone to lend it volume, or held while an intention is stated so that the stone carries it afterwards. Practitioners disagree about whether a quartz point needs clearing between uses, and the disagreement has been running for decades without resolution.
Carried forward
Pliny the Elder, Natural History, Book 37 (c. 77 CE)
Pliny reports the prevailing view that rock crystal is water frozen so hard by cold that it can no longer melt, which is why, he says, it is found in the Alps where the snow never leaves. He is wrong, and he is wrong for a good reason — the crystals really do come out of high cold places, and they really do look like ice.
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.