Boulder Opal Information

Introduction

Opals are a fascinating and varied type of gemstone with a celebrated history full of legends and superstitions to match their stunning beauty and captivating color display.

The Opal family includes such a mixed assortment of looks, textures, colors and backgrounds that it is sometimes hard to believe that they are all part of the same group.

This article is going to concentrate on the Boulder Opal, the unique native of Queensland, an Australian state famous for its tropical beaches, thick rainforests, red deserts and terrifying animals.

Boulder Opals are cut from solid rock, usually ironstone or sandstone, but the precious opal gemstone is left connected to the rock and not cut away.

The thin slivers of precious opal would be too delicate on their own or easily broken if one attempted to remove it from the host rock.

The dark-colored host rock serves as a perfect backdrop to the vibrant play-of-color by the full color spectrum of the rainbow with patterns and a brilliant fire on show.

It is very difficult to give a simple description of the colors of Boulder Opals because every single one is different. Every color of the rainbow can appear, in every possible combination, and as we look into the gemstone the colors can change and move in a phenomenon called play-of-color.

Boulder Opals are still attached to the hard rock they formed in and this gives them an additional color quality. The Opal material can be a thin layer covering a solid dark base or thin veins and patches highlighting the surface of the rock. The shimmering and glowing color of the Opal is perfectly set on this dark rocky background.

Opals can look as though they have been painted by an abstract artist who is trying out every color in his studio but where do these colors come from?

First we have the base color of the silicate mineral which is usually white but can be colored yellow, brown or red by the presence of impurities. Then we have the play-of-color effect which is created by the chemical structure of the Opal.

As the Opal forms over millions of years, layer after layer of tiny spheres harden on top of each other, creating the Opal. If these minute spheres are all the same size and the spaces between them are even then we have the right structure for any light hitting the gemstone to display the play-of-color.

The size of the spheres is incredibly small – about a tenth of a micron in diameter and a micron is 1/1000th of a millimeter. A human hair is 50 microns!

If the spheres are all quite similar in size the light can move around much more easily and we have a much better play-of-color effect and if the spheres are relatively large we will have many more red and orange flashes.

In addition to the color of a Boulder Opal we can also look at their pattern. With such a unique gemstone, a pattern may seem the wrong word but we can see a few similarities in styles such as swirling patterns, stripes, flashes and so on.

Classic Opal patterns include Chinese Writing, Broad flash, Harlequin, Floral, Peacock and Flagstone with each name being fairly self-explanatory. These patterns are very rare in all types of Opals but are more often found in Black Opals than in Boulder Opals. Boulder Opals usually have veins or patches of opal in an ironstone background so is less likely to have a wide area for patterns to show.

The more well-known patterns include:

• Banded
• Chinese Writing
• Confetti
• Flame
• Floral
• Harlequin

• Mackerel
• Palette
• Pin Fire
• Ribbon
• Roll
• Straw

Boulder Opals are basically Opals found in the Australian state of Queensland where precious Opal material is still connected to the rock in which it formed – most often ironstone and sometimes sandstone.

I know there are versions of Boulder Opal found in other parts of the world but we are concentrating on the original and, we think, legitimate source.

Within this Boulder Opal family are two quite well known varieties called Koroit Opals and Yowah Opals which are both named after the mining areas in which they are found. They both have distinctive patterns and colors that make them stand out from other Boulder Opals.

• Yowah - Opals with a delightful chocolate coloring in the surrounding rock and swirls of color
• Koroit - interesting and alluring swirls of brightly colored precious Opal on a deep brown background
• Boulder Opal Wood Fossils - formed in the gaps and crevices of wood which has fossilized into rock over millions of years creating a very unique look

When discussing the clarity of gemstones we usually mean how well we can see through the body of the stone and if there are any flaws or blemishes inside – known as inclusions in the gems trade.

Gemstones can be transparent, meaning we can look through it and see things or read words clearly and easily. Or they can be translucent where light shines through but we cannot make out shapes or images very well. Finally we have opaque where light cannot pass through even when holding the gemstone up to a flashlight or the sun.

Clarity can be graded therefore all the way from being flawlessly transparent to opaque with various grades along the way.

With Boulder Opals still being connected to the solid rock in which they formed they will, of course, be completely opaque – no light is getting through solid ironstone. The precious Opal material itself, however, can be microscopically transparent on the surface allowing us to see clearly through to the mesmerizing play-of-color.

An opal considered cloudy or milky enough to depress the play-of-color of the gemstone is certainly valued less highly.

Boulder Opals are never faceted but cut into cabochons. These cabochon cuts can be in interesting or odd shapes to capture the best colors and patterns in the rough rock. They can also be uneven or 'undulating' on the surface as the polisher reveals hidden splashes of color caught in nooks and crannies.

There are seven Chakras or energy centers throughout the body, each influencing a particular physical, emotional or mental state and each has an associated color. This philosophy came from the Hindu religion in India and was developed over 3000 years ago.

The seven chakras are as follows, Crown linked with the color purple, Third Eye (indigo), Throat (blue), Heart (green), Solar Plexus (yellow), Sacral (orange) and Root (red).

These Chakras can get blocked leading to various emotional or physical problems and need to be unblocked or cleansed in one or more of several different ways. One way to help unblock a Chakra is through the use of gemstones and crystals and depending on which color is most dominant in your gemstone will determine which chakra it will have most influence on.

Opals come in every known color so traditionally can influence any of the Chakra points and obviously an Opal that is predominantly red could help the Root Chakra while a more blue gemstone may be more useful tackling a blocked Throat Chakra.

Boulder Opals come in all the different colors too but are also still attached to the dark brown ironstone or sandstone rock in which they formed. This plus the known energy frequency possessed by Boulder Opals have lead them to be most associated with the Crown and Root Chakras but the main color can still affect their influence.

Appraising the value of opals is not an easy thing. As they are all so unique personal preference has a big impact on their value and find ten Boulder Opal valuers and I am sure you would get ten different prices. In fact that is often how gemstone merchants come up with prices for Boulder Opals.

Let’s take a deeper look at what goes into the valuation of an opal.

Number one consideration is color. Boulder Opals are almost defined by the play-of-color of the precious Opal material set against the deep background. Some stones can look fantastically vibrant at any angle so will keep a high price others will look spectacular from one angle but a bit drab from another and hence lose value.

Boulder Opals are still attached to the rock they grew in and the amount of colored Opal gemstone that appears varies from specimen to specimen. Certain gemstones will cover the entire face of the base rock while others may just have a few lines and specks. As a general rule the more coverage the more value.

Of course the color and the play-of-color will make a huge difference with a patch of dazzling red and green in a dark brown background will be more valuable than a whole gemstone of dull colored Opal.

Then we have pattern, although no two opals will look identical they do have some recognized patterns. Boulder Opal gemstones with broad flashes of color, straw-like patterns, snake skin patterns and circular swirls to name but a few styles can attract higher prices.

The base body tone Boulder Opals is important with the darker the tone the more valuable the stone. This allows the color to stand out that much more but even this is subjective as so many Boulder Opals have lighter colored backgrounds, especially the Koroit examples, and they fetch good prices too.

Now we come to the shape. Opals come out of the ground in all sorts of weird and wonderful shapes and it is the cutter’s job to make the most of this rough gem. Boulder Opals are often cut into odd shapes to capture the best of the color and pattern in the rough stone.

This was once quite detrimental to price when compared to the more regular oval shapes available in Black Opals. Nowadays with so many jewelry designers and artisans wanting to create one-of-a-kind pieces, they are getting more and more appreciation.

Boulder Opals can often have uneven or rolling surfaces unlike most Opals that are cut into smooth domes or cabochons. Again this is a technique to make the most of the colors and patterns that hide in the nooks and crannies of the host ironstone rock. This may be unwanted in most gemstones but this added texture can really intensify the Boulder Opal’s look.

Depending on to whom you listen or read, Boulder Opals either only come from Queensland in Australia or can come from Brazil, Canada, Indonesia, Mexico, Honduras and maybe a few more places. Opal material being still attached to the original rock it grew in seems to be a defining characteristic but we prefer to think of Boulder Opals as solely natives of Australia.

As far as pricing goes, Australian Boulder Opals are the most valuable if you are comparing two gemstones that are very similar in appearance.

The size of an opal can of course affect the price, all things being equal a larger gemstone will always be more valuable than the smaller version. However do not sacrifice color and clarity for the sake of size.

With Boulder Opals, be a bit wary of price per carat as most of the weight will be ironstone not Opal. Get the exact dimensions of the gemstone in question in millimeters or inches and decide whether the price is right for you.

While general Opals have been mined for thousands of years, Boulder Opals were only discovered in 1869 in the town of Blackall in the middle of Queensland, Australia although some claim Boulder Opals were first found in 1849 in Tarravilla.

They got their name because the valuable Opal is still attached to the large ironstone boulders that they formed in.

Although they were discovered over 150 years ago, the difficult terrain and vast distances to travel in this desolate area, the outbreak of two world wars, a drought and discovery of other Opal sources in Australia meant Boulder Opals did not get the recognition they deserved until the 1970s.

Since then they have slowly but surely gained appreciation and popularity throughout the world and while they are still valued less than their similar cousin, Black Opals, they are much loved in the gemstone world.

Around 100 million years ago, there was a massive shallow sea in what is now central Australia. This ocean provided the water needed to create the opals that are now found on what were the shores of this body of water. What is now known as the Winton formation is about the size of California and covers much of central Queensland and this is the location to find Boulder Opals.

It may be the size of California, but this area probably has a population of under 10,000 and the barren deserts and hills, with searing temperatures and no water make this a tough place to dig for precious gemstones.

The most famous mining sites are Yowah in the south up to Winton in the north with Quilpie right in the middle. Other spots to find this gem include:

• Koroit
• Toompine
• Kyabra-Eromanga

• Bulgroo
• Yaraka
• Jundah

With such fantastic sounding names it almost makes you want to jump in a pick-up truck full of water, food and fuel and head to the outback to visit them for yourself – almost.

The argument over how Opals were formed continues to this day and with the various gemstones seeming so different this is not all surprising. The most accepted theory for Boulder Opals is that millions of years ago water seeped into the cracks and crevices of the ironstone and sandstone rock formations of Western Queensland carrying silica material with it.

It was a much wetter place way back but as the climate changed the water evaporated away until all that was left was a silica gel-like substance which then hardened over millions of years and turned into precious opal.

In general, Boulder Opals are an untreated gemstone other than the basic cutting and polishing that they receive.

The fact that they are still connected to their mother rock can make them look a little like Opal doublets which is a treatment process where a thin sliver of Opal material is glued to a backing (made of glass, plastic or sometimes ironstone or black potch).

Doublets serve a purpose in the Opal gemstone trade but Boulder Opals are NOT doublets. (We discuss telling the difference in our How to tell a real Boulder Opal section)

Opal’s color looks better against dark backgrounds and this has lead to some treatments including smoking and sugar baths to darken the body of light colored Opals. We are not aware of this taking place very much with Boulder Opals, if at all. The ironstone in which they form is naturally dark so this is not necessary.

Another reported but not often seen treatment for Boulder Opals is reattaching the Opal material to an ironstone backing which has come loose. Resin or glue is used and it would be very tough to spot but we are not aware of this being a major problem.

Boulder Opals have a Mohs scale rating of between 5.5 and 6.5 compared to say a sapphire which is ranked very highly at 9. This makes opals about the same sort of hardness as glass so this must be considered when thinking about how you are going to use opal.

See our detailed article on the Mohs hardness scale here

Boulder Opals are still connected to the hard ironstone base in which they formed which adds a real strength to this gemstone meaning they can be used for almost any type of jewelry.

Large Boulder Opal pieces with their unique colors, interesting ironstone surrounds and odd shapes make them ideal for the latest fashion for one-of-a-kind jewelry designs.

Yes they are. Boulder Opals are the birthstones for October. With all the variety of styles and colors available there is sure to be one that would suit a loved one born in this month. Or why not treat yourself if you were born in October.

Opal is also the 14th anniversary wedding stone. A perfect gift for your partner.

At 5.5 to 6 in hardness Boulder Opals are relatively soft for a gem but the hard ironstone backing make them durable enough to survive the odd bump or knock on a hard surface however you should always take care to avoid such abuse. We always recommended you remove jewelry before physical activities, especially household cleaning, sports or gardening.

See our detailed article on the Mohs hardness scale right here

Opal Doublets and triplets involve glue and backing material so a long soak in water is not a good idea as it can lift the backing or fog up the top crystal layer but Boulder Opals are a completely natural gemstone so will not have any problems in water.

When storing your opal care should be taken to keep them away from any harder materials which may scratch, chip or break them. Keep them in individual cloth bags or boxes for extra protection.

Clean your opals using a soft cloth and only mild soap or detergent if needed.

Buying a gemstone from a reputable dealer is the best approach but this is not always possible when you are out searching for a great gemstone or a bargain!

Boulder Opals are a rather organic looking gemstone with a fair amount of rocky ironstone making up a large part of any finished gemstone. Most Boulder Opals will be a solid piece of ironstone with veins and patches of colored Opal decorating one side. Familiarize yourself with what a Boulder Opal should look like before buying.

We should mention here that there are synthetic Opals made of the same chemical composition in the laboratories and imitation opals just made of plastic. So long as they are clearly identified there is no problem. They usually are made to look like solid Opals not still connected to the mother rock so are not likely to be fake Boulder Opals.

Aurora, Kyocera and Gilson are all excellent synthetic Opal material which serve a purpose in their own way but they are not natural at all and are more likely to be replacements for White Opal.

There are composite opals, real opals attached to backing material, such as black potch, ironstone or hard plastic to simulate black opal in what is called a doublet or a protective crystal dome on the front plus a backing which is called a triplet opal.

Boulder Opals can resemble doublets in that it can have a colored Opal front and a dark stone back but this will occur naturally not artificially. From the side, doublets will show a straight line where they join whereas Boulder Opals have no obvious spot where it looks as though two separate pieces have been combined.

Price as a guide. Boulder Opals are a rare semi-precious gemstone so the price should reflect this. If you see a prime example at a price too good to be true, it probably isn’t true, so avoid buying such ‘bargains’.

This is not a complete guide on how to spot a fake gemstone but I hope it helps.

At GemSelect, we currently offer brief identification reports from your choice of two well-respected independent gemological laboratories, The Asian Institute of Gemological Sciences (AIGS) and Burapha Gemological Laboratory (BGL Lab).

This may seem a strange question when discussing opals with their incredible play-of-color displays but this is slightly different.

Certain gemstones show a distinct or dramatic change in color under different light sources. Look at an Alexandrite under electric or artificial light and it could look red, take it outside into the sunlight and all of a sudden it is green! This remarkable effect only occurs in a few gemstones, Alexandrite, Garnet and some Sapphires being the most well known but does NOT occur in opals.

Boulder Opals change color as they are viewed from different angles and to a certain extent under different light sources although if you look at some opals under fluorescent the play-of-color can be diminished because the fluorescent lights do not possess the full spectrum of colors but they are not thought of as a Color-Change gemstone.

On their own, Opals are just wonderful gemstones, the way the colors dance before your eyes is truly mesmerizing and the illusion that the display seems to be happening deep with the stone is magical.

Add to this a natural ironstone rock setting, with black, brown and golden shades creating the perfect frame for the dazzling blue, green, orange and red to be exhibited and in Boulder Opals you have some of nature’s finest work in your hands.

To be called an authentic Boulder Opal, they have to be dug from the heart of Australia’s red centre, a harsh and demanding part of our planet where water, shade and people are as scarce as the gemstones themselves.

Plucked from the earth, turned and examined in the hand, cut and polished with high expectations and perhaps a little fear, the end result can leave you breathless.