What is Opal and What is Australian Opal?
The Radiant World of Opal
Opal is one of the world’s most beautiful and precious gemstones, prized for its unique play-of-colour and natural brilliance. Alongside diamonds, rubies, sapphires, emeralds, and pearls, opal is one of the six gemstones most historically valued throughout human civilisation. The word opal originates from the Roman term opalus, which itself is derived from the Sanskrit word úpala, meaning “precious stone.” Opal is also the official birthstone for October, making it especially popular in fine jewellery and collector pieces.
Opal is a hydrated amorphous form of silica, meaning it does not have a crystalline structure like quartz. Its chemical composition is:
SiO₂ · nH₂O
This formula shows that opal is made of silicon dioxide (silica) combined with a variable amount of water, typically ranging from 3% to 21% by weight.
Key Components of Opal
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Silicon Dioxide (SiO₂):
The main component of opal, formed from microscopic silica spheres. -
Water (H₂O):
Trapped within the silica structure, water content directly affects opal’s stability, transparency, and durability.
Why Opal Displays Play-of-Colour
Precious opal displays its famous play-of-colour due to the regular arrangement of tiny silica spheres. These spheres diffract light, producing flashes of colour such as red, green, blue, and violet. Common opal lacks this ordered structure and therefore does not show colour play.
Opal vs Quartz – What’s the Difference?
Although both opal and quartz are made of silica, opal is non-crystalline and contains water, while quartz is crystalline and anhydrous. This makes opal softer and more delicate than many other gemstones.
Stability and Care Considerations
Because opal contains water, extreme heat, dryness, or sudden temperature changes can cause cracking or crazing. Proper care helps preserve the beauty of genuine opal jewellery.
Australian Opal – The Global Standard
Australia is the world’s leading source of opal, producing approximately 95% of all natural opal. This dominance has earned opal the title of Australia’s national gemstone. One of the most famous discoveries, the Olympic Australis, was found in 1956 in Coober Pedy, South Australia, and remains the largest and most valuable opal ever discovered.
Other notable opal deposits exist in Ethiopia, Nevada, Idaho, and even Mars—where NASA identified opal deposits in 2008. Ethiopian opal mainly comes from the Wollo (Welo/Wello) and North Shewa regions, with Welo opal now widely traded. In the United States, the Smithsonian Institution houses both the largest unpolished black opal, the Roebling Opal (2,585 carats), and a polished black opal known as the Black Peacock (160 carats), both sourced from Virgin Valley, Nevada.
Black Opal, Boulder Opal & Famous Opal Fields
The most important black opal field in the world is Lightning Ridge, New South Wales, with a smaller contribution from Mintabie, South Australia. Lightning Ridge black opal is considered the brightest and most valuable opal in the world, especially rare stones displaying red or multi-colour flashes on a dark body tone. These exceptional black opals are often cut directly at the opal fields and sold immediately due to their scarcity.
Boulder opal forms naturally within ironstone and is easily recognised by its dark brown backing with a thin but vibrant layer of opal on top. When expertly cut, boulder opal can display spectacular colour contrast and exceptional character, making it highly desirable for Australian opal jewellery.
Opal Meaning, History & Modern Appeal
Despite its beauty, opal gained an unfair reputation for bad luck in the 19th century after its association with misfortune in the novel Anne of Geierstein. Today, that superstition has faded, and opal remains a highly sought-after gemstone for collectors, jewellers, and those who appreciate natural, one-of-a-kind stones. With its unmatched colour play and rich history, genuine Australian opal continues to captivate gem lovers around the world.