Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opal. However, there are three notable types of opal, each with different optical effects, so the intended meaning varies depending on context. The optical effects seen in various types of opal are a result of refraction or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water in its physical structure. When the size and spacing of the silica spheres are relatively small, refracted blue-green colors are prevalent; when relatively larger, refracted yellow-orange-red colors are seen; and when larger yet, reflection yields a milky-hazy sheen.
