October’s Beauty

‘Virgin Rainbow’ Opal Literally Glows In The Dark

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The ‘Virgin Rainbow’ is one of the world’s rarest and most expensive opals. This extremely rare opal exhibits incredible fluorescence with a rainbow of different colors that make opal so distinctly unique. The opal was found in Coober Pedy of southern Australia by miner John Dunstan, working solo in the opal field. It is worth over $1 million and is now owned by the Southern Australia Museum in Adelaide.

“That opal actually glows in the dark – the darker the light, the more colour comes out of it, it’s unbelievable.” John Dunstan told ABC.

Opals are a form of amorphous hydrated silica with up to 20% water locked within the silica structure. The mineraloid, similar to a mineral but without a crystalline structure, is formed from mineral precipitation at low temperatures. The process is similar to the salt left behind when sweat or salt water dries. The complex internal structure of opal diffracts light differently depending on which angle light hits the opal. This imparts a rainbow of colors, from milky white, to red, orange, yellow, green, blue, pink, black, etc.

The Virgin Rainbow is an opalized fossil, which means the shape and initial formation of the opal replaced a fossil, likely from an ancient ancestor of today’s cuttlefish. The very generalized evolution of the opal started with an aquatic cuttlefish dying and sinking to the seafloor. As sediment buried the fish, the surrounding sediment began to lithify into rock as the shallow sea dried and became a desert. Meanwhile, silica rich pore fluids within the rock deposited opal in the cavity where the cuttlefish once remained. This is a similar process to how petrified wood is formed, whereby mineral precipitates replace organic objects.

Opals form as microscopic spheres composed of silica and other minerals, closely packed together to form a lattice framework. The order and variability in size of silica spheres determines the degree and type of diffraction of light as it passes through the opal’s structure. More regular-sized silica spheres provide more intense diffraction and are thus more desirable.

Australia has the world’s largest supply of opals, accounting for 95-97%, with most of them found in southern Australia. Of the prolific South Australia mines, Coober Pedy is the most famous, claiming many of the world’s famous opals.

 written by Trevor Nace , geologist, Forbes contributor, and adventurer.

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