Opal
Opal is known for its unique display of flashing rainbow colours called play-of-colour. There are two broad classes of opal: precious and common. Precious opal displays play-of-colour, common opal does not. Play-of-colour occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of ping-pong balls in a box. As the light waves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colours of the rainbow, called spectral colours
