An Introduction to Opals

This guest post introduces you to the different forms of opal, courtesy of our friends at Opal Galaxy. Read on to learn all about the opal in your life.

What is Opal?

Opal is a precious gemstone made primarily of hydrated silica that has been trapped within various host rocks. The hydrated silica has formed beautiful, unique colors and patterns over millions of years.

Silica is one of the most abundant minerals on earth; you might know it as being the primary component of sand and quartz. In opal, the silica has become hydrated by water to form beautiful spheres of different colours; almost similar to the way a rainbow is formed.

The hydration content of the silica in opal stone can range from 3% to 20%. Any colour formation and combination is possible. The host rocks that these hydrated silica spheres form in also varies. They can include ironstone, claystone, sandstone and even volcanic stones like rhyolite and basalt.

Unlike most gemstones, which have a crystalline structure, opal is amorphous, meaning it has no defined crystal shape. This allows the opal stone to show unique patterns and structures. This is the reason why so many people fall in love with their one-of-a-kind look.

Where Do Opals Come From?

Opals have been discovered in many different places around the world; the most famous of which being Australia. Other notable locations of discovery include Ethiopia, Mexico, Peru, America and even in some locations in the UK. 

Australian Opals

Did you know that Australia was once covered by an inland sea? This sea resulted in water seeping into the earth to form one of the most valued gemstones ever discovered: the Australian Opal.

The Eromanga Sea: an inland sea that once covered the outback of Australia. This sea existed during the Cretaceous Period.

Why are Australian opals viewed so favourably within the gemstone industry?

I believe it is their uniqueness in each gem. No two opals are the same. They are a one and only gemstone.

Australian opals do not lose their colour, fade, or change. High end quality opals from Australia are extremely rare, and because of their beauty can attract a high value within the gemstone market.

There are also different types of opals found in different mining regions in Australia. These include:

• Lightning Ridge Opals

• Black Opals

• Queensland Boulder Opals

• Coober Pedy Opals

Lightning Ridge Opals

The most famous and valuable opals in the World are mined in Lightning Ridge; an outback town inland from Brisbane. Black Opals, dark opals and crystal opals are discovered thru underground tunnel style mining efforts. Black opals are not black in colour, the name comes from their deep body tones that strike beautiful arrays of colours and flashes. The opals discovered in Lightning Ridge are a little softer than most other gemstones, so are better suited to high end, dainty style jewellery, or for simply collecting.

Queensland Boulder Opals

Boulder opals are mined by open pitt efforts with large excavator operations. The precious opal has formed within ironstone boulders, lending the name Boulder Opal. They are commonly used in everyday wearing necklaces as the ironstone is sturdy.

A popular variant of the boulder opal is pipe crystal boulder opal, which is found in sandstone boulders. These opals are typically lighter in colour and used in a lot of youth style jewellery.

A Queensland Boulder Opal: the precious opal is formed inside a sturdy Ironstone host rock

South Australian Opals: Coober Pedy

Coober Pedy is the worlds largest and biggest opal producing operation. The town is in the very centre of Australia, and is mostly built underground. White opals and crystal opals are discovered here. They are typically more common to find and a more affordable option for buyers of Australian opals.

Hydrophane Opals

Hydrophane opals are a unique type of opal known for their ability to absorb water. They are typically formed in volcanic host rocks around volcanic regions. The most famous places for these opal discoveries are in Ethiopia and Mexico. Hydrophone opals typically contain higher levels of hydration, and in some instances you can even see bubbles of water inside the opals. These opals are often viewed unfavourably within the gemstone markets as whilst they can look absolutely stunning, they can lose their colours and patterns with changes in humidity and temperature.

Other regions where opals have been discovered

Some other countries with notable opal discoveries include Peru, known for its beautiful blue, green, and pink common opals. The USA has uncovered several impressive deposits of precious opals. Honduras is known for its matrix opals, while Indonesia has produced many opalized wood fossil formations. Brazil has also yielded significant white opal deposits.

Can you find Opals in the U.K.?

Whilst gem quality opal is not found in the U.K., there have been some reports of common opal being found (otherwise known as non-precious opal that lacks color, usually referred to as potch).

Why aren’t gem quality opals found in the U.K.?

Gem quality opals are not found in the UK because the UK's geological history does not favour the slow, silica rich, low temperature processes that are required to form opals. The climate and rock types also do not suit formations of silica deposits that harden over millions of years.

Opal in Scotland

Some sources report trace amounts of common opal in volcanic rocks or weathered zones.

Wales and Cornwall

These regions have rich mining histories and complex geology, including quartz and silica-rich veins, but confirmed opal occurrences are rare and usually not of gem quality.

Devon and Somerset

Occasionally associated with old mine dumps, some silica-related mineralizations occur, but again, they are not true opals or are non-gem-quality.

<This article was submitted by Michael Shepherd from Opal Galaxy, a wholesale supplier of Australian Black Opal and Boulder Opal.>

Katriona's Thoughts on Opal Hunting in Scotland

The Stone Circle has found opal before but not in the way that you might find it at any of the more exotic locations mentioned above. We find potchy white material with little to no colour on it, and certainly not with any patterns.

We only came across it once for certain, in the hills north of Glasgow. It was a creamy white colour just like potch. Other times if we have picked it up it is because we thought it was chalcedony... We do misidentify because we are human, fallible, and not experts.

While Opal is the non-crystalline version of silica, chalcedony is the crystallised version, a type of quartz - also a silica mineral. You can tell the difference between chalcedony and opal with a hardness test, chalcedony should not scratch with a stainless steel knife but opal will.

Finally, opal stones are mined in very different conditions from what ordinary wholesale crystal suppliers bring to the table. Opal mining is fairly dangerous but they don't tend to send the kids to do it. The difference, we find, is the presence of white people in the mines.

You can read more about chalcedony in our previous blog or read about how to go rock hunting in Scotland, here.