Australian Opal are rare gemstones colorful as their country of origin. The most artistic of gems, Australian Opal’s unique character makes it one of the world’s most coveted gemstones.
Hardness 5.5 – 6.5
Refractive Index 1.370 – 1.520
Relative Density 1.98 – 2.50
Unique to Opal, ‘play of color’ is the attractive flashes of colorful brilliance that change with the angle of observation. Undeniably beautiful, Australian Opal appears to have a rainbow trapped within. The rarity hierarchy of colors is red, orange, yellow, green, blue and violet, with value lying in the brilliance, brightness and strength of colors displayed. As each Opal has its own personality and color preferences are subjective, individual tastes also play a big part. An Opal with a play of color comprising more than half of one color is named for its primary hue, while an Opal with three or more hues is called ‘multicolor’. 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). Black Opal has a black body color and may be opaque with some translucency, particularly when held to a strong light source. As Black Opal is generally more opaque, opacity is the criterion that divides black and semi-black Opal. Popular in everyday fashion jewelry, White Opal, also known as Light Opal, is translucent to opaque with a white body color. For darker colored Opal (black and semi-black) opacity demands a premium, while transparency demands a premium in lighter colored Opals (white and jelly). Because of the contrast and intensity afforded by its body color, Black Opal is the most valuable variety. Skillful lapidary is critical for Australian 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.
First discovered in 1849, Australia’s Opal fields might be the largest in the world, but only 25 percent of Australian Opal mined is gem-quality. White Opal comes from deposits near Coober Pedy (South Australia) discovered in 1915. Named for its Opal miners, ‘Coober Pedy’ means ‘white man’s hole in the ground’ and is a corruption of a local indigenous Australian phrase, ‘kupa-piti’ (boys’ waterhole). Black Opal is from deposits at Lightning Ridge (New South Wales) discovered in 1902. Australian Opal production has decreased by 50 percent in the last 10 years due to the depletion of commercially viable areas and mounting operational costs, combined with the inherent remoteness and inhospitality of Australia’s Opal fields. Despite increasing scarcity, Australia still supplies approximately 95 percent of the world’s Opals.
Durability & Care
A relatively durable jewelry gemstone, Australian Opal (Mohs’ Hardness: 5.5 – 6) 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.