How Does Opal Get Its Color? - Geology In

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.

You may have noticed that some opals don't have all the colors of the spectrum. Many opals can only display blue coloring, for example. This is because the diameter and spacing of the spheres controls the color range of an opal. Getting back to our color diffraction theory, the size and angle at which light is split determines the color produced.

Small spheres produce opal of blue color only (the most common), whereas larger spheres produce red (the rarest color). When the spheres inside the opal are bigger (about 3500 angstroms diameter) the red or orange colors are produced. At the other end of the scale, at about 1500 angstroms in diameter, the blue end of the spectrum is diffracted. Between these sizes the rest of the colors of the rainbow occur.

Therefore the rarity of the colors (most common to least common) is as follows: blue, green, yellow, orange, and red. Opals which display red can also display all the other colors of the spectrum. Therefore the possible combinations of colors in an opal can be seen as: blue only, blue-green, blue-green-yellow, blue-green-yellow-orange, and finally the full spectrum of blue-green-yellow-orange-red.

For this reason, the presence of red in an opal can greatly add to its value, since it is somewhat of a rarity. Opals can also contain aqua and purple as well as the other 'non-primary' colors which are produced when two primary colors are combined. (For example, the green and orange between the primary colors of blue, yellow, and red).