Opal:
The internal structure of precious opal makes it diffract light. Depending on the conditions in which it formed, it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the black opals are the most rare, whereas white and greens are the most common. Opals vary in optical density from opaque to semitransparent.
https://en.wikipedia.org/wiki/Opal
Structural colorarion:
Structural colouration is the production of colour by microscopically structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments. For example, peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridiscent
.
https://en.wikipedia.org/wiki/Structural_coloration
