Find out more about boulder opal!
What is boulder opal and what is its significance in the jewelry world? One of the best and often people’s favorite gemstones is OPAL. Whether it be the boulder opal pendants or rings, they seem to be the center of attention for their beauty but also get a lot of questions from collectors. What exactly is boulder opal? And why isn’t it just called opal?
Boulder opal is a rock that contains thin seams and patches of opal surrounded by or attached to its natural host rock. The cutter studies this rock and decides how to cut the best possible gem. That gem might be cut to display seams and patches of precious opal as they appear within their natural host rock. Alternatively, the gem might be cut in an orientation that presents a thin seam of precious opal as the face of the gem with its natural host rock as a backing.
Some people also consider “boulder opal” to be an ingenious cutting style that beautifully utilizes small seams and patches of precious opal that are too small to cut into gems of solid opal. Boulder opal is also known as “Natural Opal Type 2”.
Boulder Opal is a type of opal that is still connected to its host rock, which is usually ironstone or fossilized sandstone. During Boulder Opal formation, silica-rich water seeps into cracks and forms opal, when the silica spheres align perfectly you get precious opal or opal with play-of-color. The size of the silica determines the flashes of color that you see. The smaller spheres will give you blue and then you go all the way up the rainbow to the largest sphere, red. Opal actually breaks light instead of reflecting it like most other gemstones. Light entering the gem wraps around the silica sphere and causes the light to break into spectral colors.
It is impossible to remove this opal from its host rock without damaging the opal and typically the seams of opal are so thin it would not be worth the effort. Buying rough boulder is hard, you never know what is on the inside and the risk is huge.
Australian Boulder opals are the second most valuable type of opal (following black opal). Boulder opals are easily distinguished by their layer of solid brown ironstone left on the back of the stone. Boulder opals, as the name suggests, are mined from large ironstone boulders under the ground. Thin veins of colorful opal forms in cracks and fissures in these boulders. Because these veins of color are so thin, opal cutters need to leave the ironstone on the back of the opal to form a full-sized stone. The thin layer of opal in boulder opals can display any color of the spectrum in a beautiful play of color.
Although the ironstone layer providing the backing to boulder opals may appear similar to doublets upon the first inspection, the formation is completely natural.
Due to the dark backing provided by the ironstone, boulder opals generally have a dark body tone which leads to a vibrancy of color similar to that found in black opals. Sometimes the ironstone is visible on the surface of the stone, which leads to a spotted or motley appearance, and thus reduces the value of the stone. A ‘clean faced’ boulder opal, in which a whole bar of color is visible on the surface of the stone, is much more desirable and valuable than a surface displaying ironstone spots.
The thinness of the bar of colour in boulder opals means that a high cabochoned surface is extremely rare. Boulder opals usually have a flat surface or an undulating surface. Boulder opals are almost always cut in a freeform shape for this reason, to maximise the size of the stone.
In Boulder’s world, we have several types. First is Boulder Opal Matrix. This material has a lot of visible iron stone with seams of opal running through the face-up view of the gemstone.
Second is Koroite, I feel it’s best pronounced with an Aussie accent! This material looks like a painting with opal weaving between contrasting colors of ironstone.
Finally, we have full-faced Boulder Opal, where opal is covering the entire face of the gemstone. This type of Boulder Opal tends to command the highest price in the Boulder Opal world and can rival prices of the finest Black or Light Opal.
Valuing boulder opal per carat is not generally accepted in the industry due to the large component of ironstone present in the stone. However, to give a rough indication, top quality boulder opal may fetch prices of up to a high price per carat for a gem-quality stone. Clean faced stone with no ironstone spots is generally more valuable than a spotted stone. Read more about the valuation of opals.