What Gives Opals Their Colors?
The beautiful flashes of color given off by a precious opal as it is turned and viewed from different angles are caused by the various layers of the gem and by the many checks on its surface. Opals are made mostly of the elements of common sand, or silica.
Gemologists speak of an opal as hydrated silica gel, because it contains water along with the silica. The checks, or tiny cracks, in the stone occur when the water in the gel dries out. The checking and different layers of silica gel bend and reflect light rays in a shimmering play of rainbow colors.
When white light waves enter the top of an opal, they refract and bounce around inside the opal, through all the microscopic spheres and the gaps between the spheres. As the light passes through the spheres and gaps, it diffracts (splits).
Like a prism, the opal splits the white light into all the colors of the spectrum, and the light eventually bounces back out the top of the stone, at which point we get an eyeful of beautiful opal colors. The opal is the only known gemstone that is able to naturally diffract light in this way.