Gemstones and Birthstones: Oodles of Opals and Opal Categories

Opals are as unique as you are. No two opals are alike, as any jeweler who has tried to match one will tell you. While many opals simply defy description, many opals can be sorted into rough categories based on the body color of the opal and the size and shape of the color patches.

RIGHT: Matrix opal forms the heart of this brooch by Colorado cutter and jewelry designer Michael Boyd. The colors of the opal are emphasized by Boyd’s use of, among others, chrysocolla, emerald, and lapis. Courtesy Michael Boyd Studio.

Before we get started, the colors seen in opals are colloquially known as “fire,” however, the phenomenon that gives opals their fascinating flashes of color is called “play-of-color.” All of these opals may show just one hue of color or they may exhibit many as the stone is turned. With opals, usually the more colors, the merrier.

The following terms are based on the body color of the opal:

  • White opal: One of the most common commercial quality opals, these opals have a translucent to semi-translucent white body color which exhibits play-of-color. The most common mix of colors in white opals are blues and greens.

An extraordinary black opal of 9.90 carats. Photo by Mia Dixon, courtesy of Pala International.

  • Black opal: Opal with a translucent to opaque black, dark gray, blue, brown, green body color (as long as it is dark) which exhibits play-of-color. Semi-black opal (gray opal) has a lighter body color. Gray to black opal can be created by soaking a white opal in sugar, followed by a bath in acid. (Do not try this at home.) The acid turns the sugars black, and the play-of-color in the stones will appear much more vivid against the dark background.
  • Jelly opal: Opal with a colorless, transparent to semi-transparent body color that exhibits little or no play-of-color. Without any play-of-color, these stones have little commercial value.

The body color of this lovely 3.38 carat Ethiopian opal is so light and translucent the play-of-color seems to float in air. The kind of opal to get lost in. Photo by Mia Dixon, courtesy of Pala International.

  • Crystal opal: Opal that is colorless and transparent to semi-transparent, and exhibits strong play-of-color. These can be “wowser” stone as the colors seems to float in air.
  • Black crystal opal: Opal with a dark body color, that is transparent to semi-transparent which exhibits a strong play-of-color.
  • Fire opal: Opal with a yellow, red, orange, or brown body color, that is transparent to semi-transparent which may or may not exhibit play-of-color. Fire opals are often called Mexican opals. Opals with a red body color may be called cherry opals–for obvious reasons.
  • Contra luz opal: A transparent opal that shows strong play-of-color in both reflected and transmitted light. (Most opals exhibit their play-of-color only in reflected light). Contra luz is Spanish for “against the light.”

To look into this 58.84 carat Australian boulder opal is like looking into the earth’s core through swiftly hardening volcanic rock. Mesmerizing. Photo by Mia Dixon, courtesy of Pala International.

  • Boulder opal: Opal that has been cut from a thin seam of precious opal so that a backing of ironstone matrix is left on the back of the stone providing strength and durability.

The color seems to be breaking its way out of the ironstone matrix that frames the play-of-color in this 16.01 carat opal cabochon. Photo by Mia Dixon, courtesy of Pala International.

  • Matrix opal: These opals have been cut with ironstone matrix left on the top of the stone. They appeal to many people who like the rugged, hide-and-seek quality of the play-of-color.

Names for opal are also based on the patterns of color found in the stone. The names hint at some of the beauty and mystique of opal and have obviously been coined by those who have fallen under its spell: exploding flash, twinkle, Chinese writing, sunflash, starflash, rainbow, palette and peacock’s tail. The three most common names are:

  • Harlequin: Opal exhibiting a broad, angular pattern of close-set patches. Sometimes called mosaic opal. For many, harlequin is the holy grail of opals.
  • Pinfire: Opal exhibiting small patches of close-set color, like colorful stars in a white or dark background.
  • Flash: The colors in these opals play hide and seek with your vision as the stone is turned: Now you see them, now you don’t.

Because of the way it is formed—by minerals percolating out of solution—opal is known to replace the particles of buried bone, wood, shell as it dissolves over time. These are described as “opalized” bone, wood or shell. They are not considered fossils.

Opal is so hard-won and its play-of-color so rare and valued, that miners and cutters salvage whatever they can. This means that sometimes the play-of-color-bearing opal is so very thin it cannot be reliably set much less worn. This is where doublets and triplets come in.

Doublets are made by cementing a solid backing, such as black onyx, on the back of a thin slice of opal. This provides stability and durability during cutting, setting and wear. To further protect the delicate opal, a dome of quartz may be cemented to the top of the opal. This sandwiches the opal safely between a magnifying lens of quartz and a stabilizing backing of onyx. These are triplets.

While you’ll rarely come across opaque opal without any play of color, this is known to miners as potch. Potch opal may be cut if the potch itself is of an appealing color.

I’d like to credit a 1989 JCK article by Deborah Ann Hiss, “Opal: The Down Under Wonder,” and Australian Precious Opal by Archie Kalokerinos for many of these terms.

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.

Find gemstone information in every issue of Lapidary Journal Jewelry Artist!


Post a Comment