He paused to examine a long coil of hangman's rope and to read, smirking, the card propped on a magnificent necklace of opals, Caution: Do Not Touch. Cursed—Has Claimed the Lives of Nineteen Muggle Owners to Date.
For the sake of simplicity, let's look at what happens when a single color of light — green, with a wavelength of 500 nanometers — hits a precious opal.
The ordered structures that cause iridescence in opals are called photonic crystals, a kind of crystal where the bending and reflecting of light is happening in three dimensions in a periodic or repeated pattern.
This shimmering, dazzling, dancing display of light comes about from a confluence of chemistry, geology, and optics that define opals from their earliest moments, deep underground.