Gemstones & Birthstones: The Fire of Opal
Opal’s reputation depends on its display of “play-of-color,” sometimes described as “fire.” Most opal, however, does not exhibit this mesmerizing phenomenon. Play-of-color depends on a very particular set of circumstances.
ABOVE: An extraordinary opal, this 12.76 oval appears to be solid fire from one end to the other, and from front to back, something that is very rare. Photo by Mia Dixon, courtesy Pala International.
Opals are composed of microscopic spheres of silica–quartz without the quartz crystal structure. To produce play-of-color, the size of the spheres must fall into a very narrow range, between 200 nm (nanometer) and 300 nm in diameter, and be uniform in size. If the sizes are larger or smaller, the material will not exhibit play-of-color, although they may have a lovely body color. Apple-green prase opal is a lovely gemstone in its own right, even without showing any phenomena.
At one time, play-of-color was thought to be due to interference of light, the same phenomenon that creates the iridescence in butterfly wings. However, it’s now known that play-of-color is caused by a combination of diffraction and interference.
It works like this.
Waves of light bend slightly as they pass the edge of an object. This is diffraction. If two edges are close enough to form a narrow opening, the waves fan out as they exit the gap, bending around both edges of the opening. When there is more than one narrow opening, the waves of the fans begin to overlap (like the waves of the ocean when they pass through the pilings of a pier).
In opal, the silica spheres are arranged like oranges in a crate, and the spaces, or voids, between them act like narrow openings. Light diffracts through these openings as it passes through the stone. The waves of diffracted light (each a different color) overlap, and while some colors are canceled out, others are intensified. And there you have it, play-of-color.
Which colors the opal shows off depend on the size of the spheres and the voids between them. If the spheres are near the larger end of the size range, the voids are equally large and all the colors of the rainbow can appear in the opal. If the spheres are near the smaller end, color with longer wavelengths—red—will reflect. Only the shorter wavelengths of green, blue and violet will flash out.
Even though we now understand the mechanics of opal’s fireworks, the magic still remains.
Sharon Elaine Thompson is a GG and FGA who has been writing on gemstone and jewelry topics for Lapidary Journal Jewelry Artist since 1987. She also writes a line of Birthstone Romances under the name Liz Hartley.
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