Crystal
A blue light that seems to float below the surface and moves when you do.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Adularescence is the proper word for what moonstone does: a soft, billowing sheen that appears to sit a little beneath the surface and slides across the dome as the stone is tilted. Like labradorite it is a structural effect, caused by light scattering off alternating microscopic layers of two feldspars that separated as the crystal cooled. The finest stones are almost colourless with a strong blue float; the name adularia comes from the Adula massif in the Swiss Alps.
The moon association is not a modern flourish. Pliny describes a stone whose brightness waxes and wanes with the moon, and the classical world seems genuinely to have believed the light inside it was lunar. Whether the ancient selenitis was this feldspar, or gypsum, or something else again, is unresolved — the names in the old lapidaries rarely map cleanly onto modern species, and anyone who tells you otherwise is being tidier than the evidence allows.
Practice puts it with cycles: the lunar month, intuition, things that come round again. It is one of very few stones where the classical association and the contemporary one point in the same direction, which is worth noticing precisely because it is so unusual.
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 writes of selenitis as containing an image of the moon within it, which grows and shrinks as the moon does. He credits the report to Arabia and does not obviously test it, but the description of a floating internal light is a fair account of what adularescence looks like to a first-time observer.
Carried forward
Common in contemporary practice
Set out at the full moon, and by some practitioners at the new. The two customs contradict each other and both are widely taught, which is a reasonable illustration of how much of this tradition is regional habit passed on as universal rule.
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.