Crystal
A copper phosphate the colour of a clear sky, carried across more cultures than almost any other stone.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Turquoise is a hydrated copper aluminium phosphate that forms in arid, weathered rock, which is why the great historic sources sit in deserts: the Sinai, where Egyptian expeditions were mining it before 3000 BCE, and Nishapur in north-eastern Iran, whose stone reached Europe through Turkish traders and picked up the French name turquoise — Turkish stone — from the route rather than the source. The colour runs from sky blue to green depending on how much iron sits in for the aluminium, and it can shift over time as the stone takes up oils and light.
This is a stone that has to be written about carefully. Turquoise carries deep and specific significance in several Indigenous cultures of the American Southwest, and that significance is not offered for general use; it is not ours to relay, and no amount of respectful framing makes it ours. What Duskveil can honestly set out is the mineral, and the broadly shared classical and European lore — Persian and Ottoman use as a protective inlay on horse tack and armour, and the mediaeval European claim that a turquoise given as a gift held virtues a bought one did not.
A practical note that matters more than most: a large share of what is sold as turquoise is stabilised with resin, reconstituted from powder, or is dyed howlite or magnesite. None of that is necessarily dishonest, but it is often undeclared, and the price is usually the giveaway.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Pliny the Elder, Natural History, Book 37 (c. 77 CE)
Pliny describes callaina, a pale green stone brought from beyond India and prised out of frozen crags at some risk to the men doing it. He warns that oil, wine and unguents spoil its colour — an observation that holds for turquoise and is the reason jewellers still tell you to put perfume on before the ring.
Carried forward
Common in contemporary practice
The European tradition treats a turquoise as most potent when given rather than bought, and holds that it dulls when its wearer is unwell. The dulling is real and has an ordinary cause — the stone is porous and takes up skin oils — which has not stopped it being read as an omen.
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.