Why opal and moonstone glow

Turn either stone and the colour moves, because it depends on the angle the light comes in at. That's why both look different in a photo than in your hand.

Wearing them

Both are softer than most gems. Opal can crack with sudden heat or dryness, so keep it away from hot water. Take either off for the gym and for cleaning.

Opals are often doublets or triplets, a thin slice on a backing. Each piece's details say which.