A History of Opal: October’s Birthstone

The brilliantly colored opal was said to have all magic powers due to it’s play of many colors. The Roman historian, Pliny the Elder, wrote,

“In the opal, you see the living fire of ruby, the glorious purple of amethyst, the sea green of emerald, all glittering together in an incredible mixture of light.”

Opal supposedly strengthened the sight, cured eye diseases, and made the wearer invisible – making it a favorite stone of thieves. It was said to protect against contagious diseases, ease sadness, and pale in the presence of poison. Oddly, opal supposedly guards the color and life of blond hair as well.

A Quick Science Lesson:

Opals are amorphous – they have no orderly arrangement of atoms. Unlike other crystals, opals mainly will not grow in a specific crystal structure. Because of this, opals are cut to best show their beautiful colors – not specifically in a crystal structure. They are made up of tiny spheres of either amorphous silica or cristobalite. The thing that makes opals so remarkable is their play-of-color. While opals are a fairly common mineral, for it to have play-of-color, conditions in the structure must be just right. Play-of-color comes from the combination of two optical effects- diffraction and interference.

The play-of-color in these Jundah Opal earrings by K. Brunini is stunning.

Diffraction – when an ocean wave passes between pilings under a pier, it bends around them, fanning out on the other side. Light waves bend when they squeeze through small openings or pass a sharp edge. When the light bends, it also breaks up into spectral colors. This is called diffraction.

The rows of tiny spheres in an opal act like the pilings under the pier. The light wavelengths that are diffracted – and the colors we see – depend on the size of these tiny spheres. The smaller spheres cause the diffraction of shorter wavelengths which allow us to see the color blue while slightly larger spheres diffract longer wavelengths so that we see the color green as well as blue. Even larger spheres allow us to see the color red as well as the rest of the color spectrum. However, if the spheres are too large, there is no diffraction and no play-of-color.

The boulder opal in this ring by Melinda Risk displays an array of blue tones, caused by diffraction and interference.

Interference happens when light passes through layers of different materials which make the wavelengths go out of sync. This is what causes the spectral colors you see in oil or gasoline films lying on water.

Different Play-of-Color Patterns:

Broadflash or Flash: These stones show large areas of play-of-color that appear quickly across all or part of the opal as it is moved.

Pinfire: These opals show very small patches of play-of-color that change color as the stone is moved.

Harlequin: Large and distinct patches of play-of-color make this pattern the most valued.

*In general, large patches of color are preferable to small ones. But the most valuable will show the whole range of spectral colors as you rock it in your hand.*