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.

7.74 carat prase opal from Tanzania

This lovely 7.74 carat prase opal from Tanzania would make a beautiful center stone in a pendant, brooch or ring. 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.

opal, gemstones and enamels combine in this brooch by Ricky Frank

An opal is the perfect foil for the enamel work in this brooch by jewelry artist Ricky Frank, containing as it does many of the enamel colors Frank has selected. Courtesy Ricky Frank.

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).

Paula Crevoshay delights with the settings for these magnificent opals.

This 18k yellow gold ring by Paula Crevoshay exquisitely shows off this magnificent 5.50 carat black opal which is accented with tourmalines (0.80 cts total weight), and diamonds (1.02 cts total weight). Courtesy Paula Crevoshay.

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.

This Oregon opal has little to no play of color but it's beauty is sublime.

Although this 166.62 carat blue Oregon opal presents no play-of-color, its soft color and gentle translucence make it a perfect carving material. Photo by Mia Dixon, courtesy Pala International.

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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4 Comments

  1. P. Michael H at 11:11 am November 30, 2017

    Prase is not opal; it’s chalcedony.

    • Tammy Honaman at 10:47 am December 1, 2017

      FROM SHARON: Michael, you are right. When used without qualifier, “prase” generally refers to a greenish chalcedony. Robert Webster, in Gems: Their Sources, Descriptions and Identification, says this: “Chalcedony containing a multitude of hair-like crystals of actinolite produces the dull green stone called prase. Masses of the green mineral chlorite included in chalcedony produces the dark green stone known as plasma, but the material is usually so coarse that the stone is practically a jasper.” Elsewhere, in fact, he includes prase as a type of jasper. However, I’ve also heard lighter green, more translucent green chalcedony described as prase.
      Prase itself derives from the Greek work for “leek” — a light green vegetable. It’s name has been borrowed for many green stones — including chrysoprase and prasiolite. (See my Smokin’ Stones column in the March 2010 issue of Lapidary Journal Jewelry Artist, “Prasiolite,” and also Si and Ann Frazier’s informative article “Prasiolite’s Colorful History” in the same issue.)
      There are dozens of variety and trade names for opal listed in GIA’s Gem Reference Guide, one of which is prase opal, “a translucent to opaque green common opal. (Opal miners in Australia have dozens more names.) Robert Webster adds that prase opal found in Poland is colored by nickel.
      Although there are so many types of opal, I included prase opal in my list because we were fortunate enough to have Mia Dixon’s outstanding photo of an exquisite prase opal.

  2. P. Michael H at 11:14 am November 30, 2017

    Re your “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”

    That’s refraction.

    The rest of this is reminiscent of how it works, ut wrong.

    • Tammy Honaman at 10:44 am December 1, 2017

      FROM SHARON: Michael, You are right that refraction happens when light enters a transparent to translucent gemstone. When light enters any optically dense material (meaning anything denser than air—water, window glass, a gemstone), it slows down and, as a result, bends. The ratio between the speed of light in air and the speed of light in the material is the refractive index of that material. And light definitely refracts when it enters opal (RI =1.45, +.020, -.080).
      However, once light has entered the stone, it is diffraction, the bending and fanning of light as it passes through the narrow openings between the structural spheres in opal, and the subsequent interference of the different color wavelengths that causes the play-of-color in opal.
      For more information on diffraction and opal, see Gems: Their Sources, Descriptions and Identification, by Robert Webster.

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