Opalescence

Opalescence or play of color is an optical phenomenon associated with the mineraloid gemstone opal,[1] a hydrated silicon dioxide.

[2] This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues.

Each of the three notable types of opal – precious, common, and fire[3] – display different optical effects; therefore, the intended meaning varies depending on context.

The optical effects seen in various types of opal are a result of refraction (precious and fire) or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water (or air) in its physical structure.

[2][6] In a physical sense, some cases of opalescence could be related to a type of dichroism seen in highly dispersed systems with little opacity.