Crystal
One valley in Afghanistan supplied the whole ancient world with this blue, and did so for six thousand years.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Lapis is a rock rather than a mineral: mostly lazurite, with white calcite and brassy specks of pyrite scattered through it. The mines at Sar-e-Sang in Badakhshan, in the mountains of north-eastern Afghanistan, have been worked more or less continuously for six thousand years, and for most of that time they were effectively the only source the ancient world had. Lapis in a Sumerian grave, an Egyptian pectoral or a Renaissance altarpiece almost certainly came down out of the same valley.
Ground and purified, it becomes ultramarine — literally beyond the sea, from the direction it arrived. Until a synthetic version was achieved in 1826 it was the most expensive pigment in European painting, costing more by weight than gold, which is why in mediaeval contracts the amount of ultramarine was specified alongside the fee and why the Virgin's robe is blue: the patron was paying for the blue, not the theology.
Practice gives it to truth-telling and to the sort of authority that does not have to raise its voice. Some of that is the colour and rather a lot of it is the history — it is difficult to hold a stone that has been going into royal graves since before writing and feel entirely casual about it.
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's sapphirus is a blue stone sprinkled with gold dust, opaque and useless for engraving. That is lapis with its pyrite, described exactly — and it means the sapphire of classical texts is not the mineral the word now denotes, a confusion that has been quietly wrecking translations for centuries.
Carried forward
Common in contemporary practice
Worn at the throat when something true and unwelcome has to be said in a room where rank matters. The framing is deliberately regal, and practitioners are usually aware they are borrowing from the stone's history rather than from any property of it.
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.