Crystal
Hydrated silica spheres stacked in a lattice, throwing colour by diffraction — and famously unlucky, on no good grounds.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Precious opal contains no pigment at all. It is made of silica spheres, a few hundred nanometres across, packed in a regular three-dimensional array, and light passing through that array diffracts — the same physics as a butterfly wing or a soap bubble. The size of the spheres determines which colours appear, which is why red opal is rare and blue is common. Opal is also up to a fifth water by weight, and material that dries too fast can craze into a network of fine cracks.
Australia produces the great majority of the world's supply, from Coober Pedy and Lightning Ridge, where the opal fills cavities in a deeply weathered Cretaceous seabed. Ethiopian Welo opal, a more recent arrival, behaves differently: it is hydrophane, meaning it absorbs water and can go temporarily transparent if you wash it.
Its reputation for bad luck is worth naming because it is so specific and so recent. Opal was highly prized in Rome and through the Middle Ages. The superstition seems to date from the early nineteenth century and a popular novel in which a character's opal loses its fire at the moment of her collapse; the European opal market crashed and took decades to recover. It is a rare case where a piece of stone lore can be dated to a book.
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 rates opal among the most valuable of stones, holding that it gathers the fire of the carbuncle, the purple of the amethyst and the sea-green of the emerald all at once. He then tells of a senator, Nonius, who chose exile over surrendering his opal to Mark Antony and carried it with him into banishment.
Carried forward
Common in contemporary practice
The bad-luck superstition is still met with, and is still worth contradicting: it postdates the Middle Ages by centuries and appears to trace to a nineteenth-century novel rather than to any tradition. Practitioners who avoid opal for this reason are keeping faith with a publisher.
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.