An opal's story begins with silica dissolving into groundwater from the surrounding strata. Over time, some of the water was lost by evaporation through the permeable rock around it. The silica also had the natural tendency to try and reduce its contact with water. Thus, it packed together as tightly as possible, eventually developing into aggregates, forming, with the water, what is known as a 'sol'. The viscosity of this sol determined what size the aggregates needed to be before settling out of solution. Hence aggregates of similar size tended to settle together. Once these settled, they formed a gel, which gradually lost more water, and cemented together to form a hard mineral. In most cases, there was too much cementation, creating a continuous substance. However, sometimes there was just the right amount, so that the stone hardened, but there were gaps left in between the silica spheres. These gaps acted as a diffraction grating, splitting  light into its full spectrum. This is what causes the brilliant multi-hued colour play seen in precious opals.

Boulder opal is found sparsely distributed over a wide area of Queensland sandstone or boulder country where the opal fills cracks and crevices in the ironstone boulders. Opal bearing boulder is always cut including the host brown ironstone. Boulder opal is in very high demand and extremely precious. Boulder opal is usually cut to the contours of the opal vein creating a baroque wavy surface and is often freeform and irregular in shape, making boulder opal unique and exclusive among it's peers.

Precious opal is the valuable form of opal which shows colourful patterns that result from the diffraction of light in a process similar to how the colours of a rainbow are formed.  Good quality opals will show some variation in their patterns and colours as they are viewed from different angles, for example when they are turned in the hand.

Potch, or common opal, is generally of no value compared to precious opal as it does not show any diffraction of colour.  Common opal occurs far more abundantly in the opal mines than precious opals.

‘Yowah Nuts’ are a sub species of Boulder Opal, they are typically small boulders and are found in a unique formation consisting of a regular band containing nuts packed in a conglomerate. The opal is found either as a central kernel, or as thin veins, surrounding an ironstone centre or traversing the nut, without reaching the outer edge.
Yowah is Queensland’s southern most opal field and is centred on the small town
of Yowah 40km north east of Eulo and 150km west of Cunnamulla. Koroit is an associated opal field 80 kilometers east of Yowah .
Once the nuts have been removed from the ground they must be cleaned, sorted and usually sawn, so as not to damage the opal. 
Yowah Nuts are mostly 1.5 to 3.5 centimetres in diameter, but they may be the size of a fist . The vast majority of these small ironstone concretions are either solid or have hollowed centres and do not contain any opal.

Crystal opal is transparent and is pure opal (hydrated silica.) It typically has sharp clarity of diffracted color visible from within and on the surfaces of the opal. When held out of the direct light, crystal opal displays some of the most intense opal color. This is the type of opal used in opal inlay jewellery, which has the base of the setting blackened before the precision cut crystal opal is set into it.

White opal is the most common type of precious opal  with a milky appearance which dominates the diffracted colors. All of the opal fields produce white opal with most of it being mined in Coober Pedy.

Koroit is located approximately 1000 kilometres from the east coast of Australia, right in the heart of Outback     Queensland. The opals from here are unlike opal anywhere else in the world. 

Koroit renowned for the beautiful and intricate patterns of precious and common opal that run right through the host rocks. Sometimes they are called "Aboriginal Art Opals" due to their resemblance to native Australian paintings. Even some stones with only common opal (potch) can be collectors pieces, with the patterns in the stone forming beautiful pictures. Picture stones are in high demand world-wide.

The area is also famous for its opal "Nuts". These are ironstone concretions that occasionally contain deposits of precious opal. About one Opal Nut in every ten thousand has a centre of crystal opal.  Others have opal embedded into the shell of the Nut, with opal patterns running in concentric circles from the inside to the outside of the Nut. 

Koroit along with Yowah opal is in great demand world-wide and makes beautiful jewellery or unique collectors pieces.


The colours and patterns of Yowah and Koroit are hypnotic. Once you own one opal you will want to own more.

Opal is currently being mined from depths between 1 metre (3 feet) and 30 metres (90 feet) below the surface.  The opal is found in sedimentary rocks that were deposited in a freshwater environment during the Cretaceous period, about 110 million years ago. These opal fields may be isolated or occur in groups.  The main opal field "groups" are located at Lightning Ridge, Coocoran, Grawin/Carter's, Glengarry/Sheepyard, Wyoming, Jag Hill and Mehi.

A number of theories, or models, exist for how opal was formed in the Lightning Ridge region and all the other fields.