Queensland Boulder Opal are uniquely attractive Opals from sparse deposits spread over a huge area of the Australian state of Queensland. Prized for its contrasting colors, Queensland Boulder Opal’s popularity has skyrocketed over the last 20 years.
Hardness 5.5 – 6.5
Refractive Index 1.370 – 1.470
Relative Density 1.98 – 2.50
Created when Opal fills cracks and crevices in ironstone boulders, Queensland Boulder Opal is a very unique variety with attractive ‘play of color’ flashes sparkling on the surface of the contrasting ironstone host rock, which also provides excellent durability. Unique to Opal, ‘play of color’ is the attractive flashes of colorful brilliance that change with the angle of observation. Opals are classified by their host rock (also known as ‘potch’ or ‘matrix’) on which Opal forms and their resulting transparency and body color (the base color on which Opal’s color play is visible). While the GIA (Gemological Institute of America) classifies Boulder Opal with visible ironstone as ‘Opal with matrix’ and calls Boulder Opal with no ironstone visible ‘Opal in matrix’, most Boulder Opal includes the host chocolate brown ironstone. This results in striking, irregular patterns, making each gem unique. Skillful lapidary is critical for Queensland Boulder Opal and each gem has been cut with a desirable finish, proportion, shape, and symmetry.
The formation of Australian Opals starts 140 million years ago at a huge inland sea’s geological feature called the ‘Great Australian Basin’. The majority of Australia’s Opal fields are located in the basin and were formed from the weathering of sandstone deposited over older host rock. The structure of Opals is unique and comprised of tiny spheres of silicon dioxide forming a pyramid shaped grid, interspersed with water. It’s the refraction of light through the spaces between these spheres that produces Opal’s characteristic play of color. For example, the rare reds are reflected from the scarcer smaller spheres, while Opal’s blues are reflected from the more common larger spheres. Opal without play of color has its silicon dioxide spheres more randomly arranged. Apart from their colorful brilliance, Australian Opals are also valued for their hardness and stability, a key consideration for a gem containing three to six percent water.
You’re probably already aware of some classic Australian slang like ‘G ’Day Mate’, but unless you’ve visited an Opal field, terms like ‘knobby’, ‘seam’, ‘floaters’, ‘noodling’ and ‘ratters’ are likely unknown. A knobby is a round fist-sized ball formation of Opal, while a seam are Opals in flat layers like a sandwich. Floaters are Opal that is visible from the surface and typically indicative of an underground deposit; noodling is hunting through old mine tailings to find Opal that others missed; and ratters are people who poach Opal from another’s claim – a big no, no that was once arbitrarily and severely punished!
One of October’s birthstones, Opal is from the Greek ‘opallios’, meaning ‘to see a change’. While Opal has been Australia’s national gemstone since 27th July, 1993, this gem has been part of indigenous Australian culture for over 60,000 years. Dreamtime legends passed down by storytellers’ call Opal the ‘fire of the desert’, linking it to creation myths. According to tribal folklore, their ancestral creator came to earth on a great rainbow, which turned the rocks it touched into resplendent Opals, colored with the hues of a rainbow.
Australia’s Opal fields might be the largest in the world, but only 25 percent of Australian Opal mined is gem-quality. Discovered in 1869, Queensland Boulder Opal occurs sparsely over a huge area of the Australian state of Queensland centered on the town of Quilpie (pronounced Kwill-Pee), but extending to Winton in the north and Cunamulla in the south. Recent years have seen dramatically lower production levels for Queensland Boulder Opal with output decreasing by 50 percent in the last 10 years due to the depletion of commercially viable areas and mounting operational costs.
Durability & Care
A relatively durable jewelry gemstone, Queensland Boulder Opal (Mohs’ Hardness: 5.5 – 6.5) should always be stored carefully to avoid scuffs and scratches. Clean with gentle soap and lukewarm water, scrubbing behind the gem with a very soft toothbrush as necessary. After cleaning, pat dry with a soft towel or chamois cloth.