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Gem Profile Oct. 28: Boulder Opal
by Dale "Cougar" Armstrong, Wire-Sculpture.com
Daily Wire Jewelry Making Tip for
October 28, 2011
one of a series, Mystifying Opals
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
Throughout this short series about opals, we have learned a lot of basic information about this mystifying stone. That most common opal does not have a play of color, but because it is very abundant, rather soft, and is found in a wide variety of colors with an internal glow, it makes lovely carvings. And that some “common” opal really isn’t so common, such as beautiful Mexican jelly opal and the wonderful blue and pink opal from South America; all of which work into lovely cabochons. Rare, precious black, and chocolate opals can be found in specialized locations, and have amazing play of color; milky white opals are the most well known. We also learned that opalized fossils are actually the remains of prehistoric life that have been either been coated or filled with opal material, making these specimens even more special, and that crystal or clear opal can be faceted. But probably the most popular opal used by wire jewelry artists is boulder opal.
Resembling precious black opal, it is not surprising that the only location in the world to find boulder opal is in Australia. As we have learned, opal forms in unusual places, often very thin and in Queensland, this thinner opal formed on a matrix of an iron rich sandstone called ironstone. To break up this bedrock, heavy excavation processes are used that results in huge chunks of ironstone, or boulders. Now the definition of a boulder refers to a huge, independent mass of rock that has been weathered so the edges are rounded, however this has been modified to cover rough edged stones that have been “detached from their place of origin.” Besides the amazing array of colors that play in the opal that formed on this ironstone, this variety does not have a huge water content, making it a very stable form of opal that is not apt to craze or crack as it ages. When the opal is veined throughout the ironstone, it is called matrix opal.
Boulder opal is difficult to cut into domed cabochons, due to the way the opal seams and veins form within the ironstone matrix, so most boulder opals are available in freeform, flat shapes. Most often a stone is cut, hoping that most of the opal remains on one side of the matrix, but if the opal splits down the middle (called a boulder opal split) it can be mirrored on each sided, which a lapidary can make into matching earring cabochons. Boulder opal can be classified in several categories, according to the color of the matrix rock: black, crystal, and light. The play of color in a boulder opal can include any combination of red, orange, green, blue, purple, and even pink!
Although boulder opal may seem to be relatively new to the gemstone market, it was actually first discovered in the 1870s. When precious black opal was found in New South Wales in 1903, boulder opal was all but forgotten about until around the 1980s. Sometimes one can become confused when shopping for boulder opal, because ironstone is often used as the backing for doublets and triplets made of either precious black or white opal. Please be aware that true boulder opal should be sold by the piece, and not by the carat or gram weight.
Yowah Nut and Koroit Opals
A specialized type of boulder opal is called Yowah Nut. Found specifically in the opal fields of Yowah, Australia, this is where pockets of crystal clear opal, loaded with electric colors, formed within hard conglomerations of clay and ironstone. When the matrix rock is cut away to expose the pocket, the result resembles a nut. To view an excellent specimen of a Yowah Nut, please visit this page of The Smithsonian, department of Mineral Sciences.
One of the other well known boulder opal fields in Queensland, Australia is Koroit. I found this blog by Gene McDevitt, Koroit Opal News, to be an extremely interesting way to learn more about how this boulder opal is mined and then cut. He also includes excellent photos of the wildlife that share their habitat with opal miners.
As promised, next week I will tell you a bit about one of my personal favorite gemstones, Ammolite!! Have you ever wire wrapped ammolite before? Email pictures to firstname.lastname@example.org, and they could be featured!
- 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
- The World of Opals by Allan W. Eckert, ISBN 0471133973
Gem Profile by Dale "Cougar" Armstrong
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