The name “opal” comes from the Latin opalus, meaning “precious stone,” and ancient Romans thought of opal as the gem of love and hope. The ancient Roman writer Pliny observed that it is as if the flaming reds of ruby, the magnificent greens of emerald, the golden yellows of topaz, the deep blues of sapphire, and the rich purples of amethyst become one within opal.

Like quartz and agate, opal is primarily composed of silicon dioxide, but it also contains 3 to 10 percent water by weight. Unlike most other gems, it is an amorphous (non-crystalline) mineral. Created slowly by the forces of nature between 120 million and 30 million years ago, opal occurs in abundance near the surface of the earth. However, stones showing the beautiful rainbow of sparkling color are scarce: Each of these is a rare and valuable gemstone with its own unique character.

The cause of the sparkling spectral colors (play-of-color) in opal was not understood until the 1960s. Opal, which appears fine-textured and homogeneous to the naked eye, is actually an aggregate of extremely tiny spherical or irregular particles. Opals that exhibit play-of-color are made up of spherical particles of uniform size that are packed together in an orderly fashion.

Depending on the size of the particles, diffraction occurs, resulting in play-of-color. When the particles are small, purple can be seen; when medium-sized, green; and when large, red. Light opal is often called “white opal,” but as a general rule this is not correct. “White opal” should be used to describe opals with a white or milky body color, and this does not apply to the highly transparent crystal opal.

The light opal in this photograph is one of gem quality, exhibiting bright, vivid colors. Gem-quality opals will show these bright, vivid colors in an attractive pattern, and have a good shape. Within gem qualities, those showing strong red colors will command a premium, with strong oranges and greens following in order of desirability. Opals that are cloudy or dull are of low quality, regardless of the colors they exhibit.