Gem Profile- Black Precious Opal
This article is one of a six-part series on Opal. Here is a complete list of our Opal articles: An Opal Introduction, Common Opal, Black Precious Opal, White Precious Opal, Opalized Fossils, Boulder Opal
When most people hear the word "opal," they imagine a white or black stone flashing with a spectrum of colors known as precious opal.
As discussed in our Mystifying Opal Series, Part 2: Common Opal, the play of color in opal determines its value.
This effect occurs because the stone contains uniform silica microspheres. As light passes through them, it is split, creating opalescence.
June Culp Zeitner aptly described precious opal as "crushed rainbows."
A few examples of precious black opal cabochons. Private collection, Dale Armstrong.
As discussed in our Mystifying Opal Series, Part 2: Common Opal, the play of color in opal determines its value.
This effect occurs because the stone contains uniform silica microspheres. As light passes through them, it is split, creating opalescence.
June Culp Zeitner aptly described precious opal as "crushed rainbows."
A few examples of precious black opal cabochons. Private collection, Dale Armstrong.
The base silica of precious opal can be black, gray, blue, or white.
Some of the most striking stones are black opals from Lightning Ridge in New South Wales, Australia, which typically have backgrounds ranging from black to gray.
Opal base colors can also include red, orange, yellow, blue, green, and brown, usually with one dominant color and a secondary color.
Cynthia B. Reuschel explains, "If a stone has a dominant blue and secondary green, it has a blue-green fire.
If it has other strong fire colors, it is a blue-green multicolor. Red fire is generally the most valued color."
Some of the most striking stones are black opals from Lightning Ridge in New South Wales, Australia, which typically have backgrounds ranging from black to gray.
Opal base colors can also include red, orange, yellow, blue, green, and brown, usually with one dominant color and a secondary color.
Cynthia B. Reuschel explains, "If a stone has a dominant blue and secondary green, it has a blue-green fire.
If it has other strong fire colors, it is a blue-green multicolor. Red fire is generally the most valued color."
The geology of Lightning Ridge consists of sandstone and conglomerate layered over weathered claystone.
Opal is found in the claystone layer, known as "opal dirt," at depths of 20 to 59 feet (8 to 18 meters) in seams and rounded nodules called "nobbies."
This opal dirt is as hard and dry as concrete.
Although native Aborigines knew about opal for centuries, the Australian Department of Mining records the first documented stones from Lightning Ridge in 1873.
Opal is found in the claystone layer, known as "opal dirt," at depths of 20 to 59 feet (8 to 18 meters) in seams and rounded nodules called "nobbies."
This opal dirt is as hard and dry as concrete.
Although native Aborigines knew about opal for centuries, the Australian Department of Mining records the first documented stones from Lightning Ridge in 1873.
Black opals in Lightning Ridge are mined using two main methods.
One method involves sinking a shaft and digging tunnels to extract seams and nobbies, while the other method is open-pit mining, where the area is fully excavated.
While researching Australian mining, I found the website of a miner who is also a photographer.
His gallery, Ridge Lightning, features stunning images of tunnel mining at Lightning Ridge and showcases some of the opals discovered there.
A black opal that may not make it to a final polish unless the potch vugs can be worked out.
One method involves sinking a shaft and digging tunnels to extract seams and nobbies, while the other method is open-pit mining, where the area is fully excavated.
While researching Australian mining, I found the website of a miner who is also a photographer.
His gallery, Ridge Lightning, features stunning images of tunnel mining at Lightning Ridge and showcases some of the opals discovered there.
A black opal that may not make it to a final polish unless the potch vugs can be worked out.
Top-grade precious black opal is extremely rare.
A stone's value depends on several factors: base color, brightness of fire, fire color, clarity and inclusions, consistency of color across the stone, and "directionality," which measures whether the colors are visible from only one angle.
The finest Lightning Ridge black opals sell for $15,000 to $20,000 per carat.
Can you imagine having 17 ounces of high-quality material and selling it for just $30? You'll have to read *Lightning Ridge Opal* by gemstone expert David Federman to find out how that happened.
Although Lightning Ridge is considered the "capital" of precious black opal, another Australian source is Mintabe.
A stone's value depends on several factors: base color, brightness of fire, fire color, clarity and inclusions, consistency of color across the stone, and "directionality," which measures whether the colors are visible from only one angle.
The finest Lightning Ridge black opals sell for $15,000 to $20,000 per carat.
Can you imagine having 17 ounces of high-quality material and selling it for just $30? You'll have to read *Lightning Ridge Opal* by gemstone expert David Federman to find out how that happened.
Although Lightning Ridge is considered the "capital" of precious black opal, another Australian source is Mintabe.
Some people may have heard of Honduran black opal.
Its basalt matrix is usually dark gray rather than deep black, and most of the opal formed in very thin seams, so it is often sold as a doublet or triplet.
A Honduran matrix opal can look like black paper sprinkled with multi-colored glitter.
Some volcanic material contains tiny vugs, and the opal that fills them forms pinpoints of color. These colors cover the entire spectrum, but much of the rough is difficult to polish.
The challenge comes from pockets of "potch," a soft, powdery pumice that is tricky for lapidaries to work around something that has been troubling my husband for years.
What Honduran matrix opal can look like, but beware of sugar treatments!
Pre-formed Honduran black opal. Yes, in my studio we have been experimenting with different stabilization methods, so hopefully this material will one day be polished!
Its basalt matrix is usually dark gray rather than deep black, and most of the opal formed in very thin seams, so it is often sold as a doublet or triplet.
A Honduran matrix opal can look like black paper sprinkled with multi-colored glitter.
Some volcanic material contains tiny vugs, and the opal that fills them forms pinpoints of color. These colors cover the entire spectrum, but much of the rough is difficult to polish.
The challenge comes from pockets of "potch," a soft, powdery pumice that is tricky for lapidaries to work around something that has been troubling my husband for years.
What Honduran matrix opal can look like, but beware of sugar treatments!
Pre-formed Honduran black opal. Yes, in my studio we have been experimenting with different stabilization methods, so hopefully this material will one day be polished!
Did you know that an opal similar to Honduran can be found in the state of Louisiana? Some of it has a dark background and some is almost blonde in color. The stories about Louisiana opal vary from source to source, my favorite is in a past issue of Rock 'n Gem magazine, around 1995.
The United States also boasts some fine, precious black opal that rivals Lightning Ridge material! On Rainbow Ridge in Virgin Valley, Nevada, a dazzling dense black opal is mined.
The United States also boasts some fine, precious black opal that rivals Lightning Ridge material! On Rainbow Ridge in Virgin Valley, Nevada, a dazzling dense black opal is mined.
Discussing darker opal backgrounds wouldn't be complete without mentioning "chocolate" opals.
These precious opals from Ethiopia have rich dark and milk chocolate colors, and modern gem enthusiasts didn't take notice until the late 1990s. Before then, Ethiopian opals were considered unstable and prone to crazing.
Lapidaries now understand how long an Ethiopian opal must cure after being dug before it can be safely cut.
Chocolate opal backgrounds can range to brownish-peach and display electric color play in green, blue, orange, yellow, and red, with some flashes of neon indigo or turquoise.
Ethiopian opal forms in nobbies encased in rhyolite. Most of these geode-like formations contain only potch, with roughly 20% showing play-of-color, making triple-A grade Ethiopian opal rare and valued up to $200 per carat for cabochons or faceted stones.
Interestingly, archaeologists in Kenya found early tools made of this opal, suggesting it may have been mined in Africa before Australia, though the full history remains uncertain.
Very difficult to photograph, these are precious dark chocolate opal cabochons (front) and "root beer" opal rough (back) from Ethiopia. Private collection, Dale Armstrong.
These precious opals from Ethiopia have rich dark and milk chocolate colors, and modern gem enthusiasts didn't take notice until the late 1990s. Before then, Ethiopian opals were considered unstable and prone to crazing.
Lapidaries now understand how long an Ethiopian opal must cure after being dug before it can be safely cut.
Chocolate opal backgrounds can range to brownish-peach and display electric color play in green, blue, orange, yellow, and red, with some flashes of neon indigo or turquoise.
Ethiopian opal forms in nobbies encased in rhyolite. Most of these geode-like formations contain only potch, with roughly 20% showing play-of-color, making triple-A grade Ethiopian opal rare and valued up to $200 per carat for cabochons or faceted stones.
Interestingly, archaeologists in Kenya found early tools made of this opal, suggesting it may have been mined in Africa before Australia, though the full history remains uncertain.
Very difficult to photograph, these are precious dark chocolate opal cabochons (front) and "root beer" opal rough (back) from Ethiopia. Private collection, Dale Armstrong.
Resources
Print Resources:
Gemstones of the World by Walter Schumann, ISBN 0-8069-3088-8
Minerals of the World by Walter Schumann, ISBN 0-8069-8570-4
Opals by Fred Ward, ISBN 1887651047
Simon & Schuster's Guide to Gems and Precious Stones by Curzio Cipriani and Alessandro Borelli, ISBN 0-671-60430-9
Internet Resources:
www.dpi.nsw.gov.au/minerals
www.ridgelightning.com
Print Resources:
Gemstones of the World by Walter Schumann, ISBN 0-8069-3088-8
Minerals of the World by Walter Schumann, ISBN 0-8069-8570-4
Opals by Fred Ward, ISBN 1887651047
Simon & Schuster's Guide to Gems and Precious Stones by Curzio Cipriani and Alessandro Borelli, ISBN 0-671-60430-9
Internet Resources:
www.dpi.nsw.gov.au/minerals
www.ridgelightning.com
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