Science & Environment

Dazzled by Australia's precious opals

Precious opal Image copyright Thinkstock
Image caption Opal is a hydrated amorphous form of silica that fills cracks in its host rock

It's exactly 100 years since the teenage Willie Hutchinson stumbled across a few pieces of opal while walking in the Australian Outback.

There with his father to prospect for gold, the youngster's chance find led to a gemstone mining boom and the establishment of the town of Coober Pedy.

Today, the settlement even refers to itself as the "opal capital of the world", with the wider Australian deposits producing more than 80% of the world's precious opals.

Coober Pedy's gemstones are known for their clear or whitish colour, but some will dazzle like a rainbow. Experts talk of "church windows" to describe examples that mimic stained glass.

Already legally defined as Australia's "National Gemstone", the precious opals could be in line for a new status - that of Global Heritage Stone Resource (GHSR).

This is a designation being devised by an international group of geologists. The intention is to name and define those stones deemed to have particular significance in human culture.

Image copyright Thinkstock
Image caption 2015 is an important year for Coober Pedy and its opal ties
Image copyright Thinkstock
Image caption Tiny spheres of silica stacked inside opal split up the light into dazzling rainbow colours

The initial concept was to recognise the classic rocks used in building construction, such as Portland stone, Carrara marble, Sydney sandstone, and granites like Norwegian larvikite.

But there are also many famous ornamental and precious stones that could claim equally significant heritage, and as a consequence the geologists now find themselves examining the limits of qualification.

GHSR status - no stone has yet been designated - will be important for architects and conservators when they have to restore old buildings, for example.

It will make it easier for them to stipulate very specific materials, confident in how they will perform and weather over time.

It may also offer some protection to the quarries and mines from which the designated stones are sourced.

If nothing else, GHSR status would be a marketing fillip.

Only those marbles from Tuscany in Italy could rightfully lay claim to the name "Carrara"; "Welsh slate" would be just that; and, if awarded the status, only the precious opals from Down Under could say they were "Australian".

Image copyright Thinkstock
Image caption Carrara marble: GHSR designation was originally intended for the classic dimension stones

But not all geologists are happy with the idea of the gemstones' inclusion.

For these critics, the opals, with their limited supply and wide-ranging use in jewellery, are deemed to be too far removed from the original spirit of the initiative, which was to promote the classic dimension stones - those cut or shaped to be used for building.

There is a sense, also, in which they are too "processed". And, in any case, the detractors argue, the opals are so diverse in form and colour that it will actually be very difficult to give them a clear and tight description.

Barry Cooper, on the other hand, believes his national gemstone just scrapes under the bar. The South Australian is the secretary of the Heritage Stone Task Group of the International Union of Geological Sciences.

"Some of my colleagues are up in arms," he conceded.

"Where's the limit? If you ask me, I'd say stones like diamonds and sapphires are far too manufactured. But stones like opal are not only jewellery gemstones, they can also be used in sculpture and mosaics - they broach across into art. And that gives them a deeper cultural significance.

"The crux is that there is probably some value in them being designated."

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Media captionDr Barry Cooper: "Some of my colleagues are up in arms"

The Coober Pedy centenary certainly speaks to the opals' heritage. And Dr Cooper is quick to recall that Queen Elizabeth, the first reigning monarch to visit Australia, in 1954, was presented with an opal necklace from the South Australian Government. Opals are iconic, he says, and the ongoing mining means there would unquestionably be commercial and community benefits from the broader international awareness that came with designation.

What may help is the geological context. Australian precious opals result from very special conditions that pertained 100 million years ago when the great Eromanga Sea, then covering central Australia, started to dry out.

Highly acidic fluids dissolved silica from quartz-rich sandstones that then later precipitated as precious opal.

These circumstances are quite different from some of the world's other opal fields, which may trace their origin to a volcanic setting, not the sedimentary one seen in Australia.

International kudos

Whatever its merits, the gemstone will have to wait its turn.

The Global Heritage Stone Resource project formally got under way in 2008. Only seven years later are geologists close to acknowledging their first designation.

This is likely to go to Portland stone, the redoubtable English building block quarried in Dorset.

The creamy-grey limestone has become, literally, the bedrock of the British establishment, used in the construction of Buckingham Palace, St Paul's Cathedral, the Bank of England, much of Whitehall - even the BBC.

Michael Poultney is the MD of Albion Stone Plc, its biggest producer.

"I see the designation as recognising those stones that have made a massive contribution to the built environment, not just locally but internationally as well," he told BBC News.

"Portland stone has been used across the world - in Belgium and in Holland, and even in New York in buildings such as the UN.

"Getting the designation would mean a huge amount to us on the international market. Foreign buyers assume it's not available in commercial quantities and we're trying to dispel that myth. Designation would also give us kudos."

Barry Cooper was speaking at a meeting of heritage stone experts at the recent European Geosciences Union General Assembly.

Image caption The BBC's HQ is one of the many buildings in London that incorporate Portland stone
Image caption Fossil record: Portland stone is a limestone of late Jurassic age that was formed in a warm sea and follow me on Twitter: @BBCAmos

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