Treasures from Down Under

For those of you who've been involved with opals and gemstones for many years, here you can chat with your peers.

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PinkDiamond
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Treasures from Down Under

Post by PinkDiamond »

From Jewellery Business, here's a good one on these authors' visit to Australia and it's various opal mining areas. I'll give you page 1, and you can use the link to reach the other 2 pages. Enjoy! :)

Treasures from Down Under
December 3, 2018
By Benedicte and Martine Lavoie


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All photos courtesy Pierres de Charme

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A 4.11-carat black opal from Lightning Ridge, Australia.

"We recently returned from a trip to Australia, the land of opals and kangaroos! There, we had the opportunity to visit several opal mines and had the pleasure of meeting with extraordinary miners, who were generous enough to show us their mines and explain the opal extraction process from A to Z.

Most mines are on private land and are rented to miners as concessions. These lands are extremely arid and mostly used for livestock. All the miners we met proved to us their work relies on patience, determination, and a certain dose of madness.

Years in the making
Opals need extremely special conditions and millions of years to form. The following is a very brief and simplified overview of these gems’ formation—a process beginning in river waters that flowed when dinosaurs still lived on Earth.

This river water, already rich in silica, would flow on sandstone beds. With gravity, the water-and-silica solution was deposited in cavities in the form of a gel, and the water slowly evaporated. The remaining silica, transformed into miniature spherules of about 150 to 700 nanometres in diameter, became trapped in the cavities. (This is why opals are often found in the form of nodules or horizontal ‘veins’ corresponding to ancient riverbeds and inland seas.)

If the silica-rich molecules form into regular spheres and ordered rows, the light diffracts, which produces a colour scheme. If they are simply deposited in no order, the result is the common opal or ‘potch,’ which forms without any colour scheme. The largest (and much less common) spherules produce red colour sets, while smaller ones produce purples and blues. Black opals showing red and orange flashes are rarer and thus tend to be more expensive. ... "

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OpalSpectrum
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Re: Treasures from Down Under

Post by OpalSpectrum »

This river water, already rich in silica, would flow on sandstone beds. With gravity, the water-and-silica solution was deposited in cavities in the form of a gel, and the water slowly evaporated. The remaining silica, transformed into miniature spherules of about 150 to 700 nanometres in diameter, became trapped in the cavities....


we still don't know/understand how opal form but in the last InColor issue 41 they say opal form different way
- not from surface water (rich in silica) which go down and is ''..deposited by bottom-up gravity-driven sedimentation of colloidal silica spheres (i.e. the traditional model) but rather was formed in very vigorous opal fluid flow hydrodynamic regime .." ''... which is the way in which most mineral veins are commonly thought to have been formed in the Earth's crust''
which mean that silica rich fluids are pushed up by pressure to the surface

to me this make more sense although I don't agree with everything they claim
:roll:
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mick
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Re: Treasures from Down Under

Post by mick »

To me, the real clue, despite all the experts, is in the very fine black volcanic ash that is in black potch and I believe, helps settle the "opal layer".
Ive seen black Opal now from every field of Australia. It is possible. How did that black volcanic material travel from QLD to SA? was it wind or rivers? or devastating pyroclastic event that covered the landscape with black ash?
I think white opal is whats left after the black opal has flowed south.
No one will ever know.
Thats half the fun.
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