Opal is perhaps one of the most popular phenomenal gemstones that shows the phenomena play-of-color. And this phenomena can only be used to describe natural opals and synthetic opals.
Play-of-color in an opal can be defined as the pseudochromatic optical effect resulting in flashes of colored light as light ray strikes the gemstone.
Opal does not have any orderly arrangement of its internal atoms. It is not crystalline but its structure is amorphous, that is, it does not have a clearly defined shape or form. Its internal structure is made up of tiny spheres of amorphous silica stacked together in a grid-like pattern. This stacking of the little spheres gives the effect of play-of-color.
When light passes through an opal, the rows of spheres diffract the light rays and scattered it. The diffraction of shorter wavelength of light in smaller spheres will give rise to blue, while the diffraction of longer wavelength as in larger spheres give rise to red.
Opal also has a layered outer surface, which contribute to the optical effect of light interference. Interference occurs when light ray passes through different layers of different material.
Hence, it is the interplay between diffraction and interference of light that gives opal the play-of-color phenomena.
The most expensive and sought after is the black opal where the intense spectral colors seem to explode from a vivid and rich black background. Flashes of red in a black opal is also the most valuable of the opal gemstones.
Opal is the national gemstone of Australia. Nearly 97% of the world’s opal comes from Australia.