"Rainbow Bones" - How are they formed?

21 February 2013

Opals are highly sought-after rainbow precious stones that are showcased in jewellery or decorative art.

In addition to their aesthetic appeal, opals play a very significant role in science.

Australia produces 95% of the world's precious opals and they are formed in sedimentary rocks. The remaining few are usually found in volcanic areas around the world.

South Australian Museum research has looked at what leads opal to form in wood and bone, and in some cases, ancient dinosaur fossils. The beautiful spectrum of colours is found on many valuable historic specimens in Australia, but several questions remain about how and why opal forms on fossils. The Museum has a stunning collection of opalised fossils and continues to study what creates these treasures.

The Museum is also attempting to acquire a beautiful and rare opalised belemnite. A belemnite is an ancient, squid-like creature over 100 million years old. Its hard parts, originally made of calcium carbonate, have been fossilised over time as crystal opal replaced the carbonate. The acquisition would represent one of the most valuable and breathtaking stories of nature that South Australia has to offer.

The specimen (part of a belemnite) is a 72.64ct stone of unsurpassed quality and is without any doubt the finest single opal to come from the State. It is unique in its quality and has been independently assessed by two accredited appraisers with a conservative insurance valuation of $1 million, however the owners have kindly offered a reduced price to the Museum.

The stone was found in September 2003 at Brown's Folly, Coober Pedy, South Australia and has been in the finders' possession since. It measures 63.3 x 13.3 x 14.3 mm and is composed of extremely rare black crystal opal with large patches of full spectral colour, against a natural dark background. Not only is it the finest stone to be recovered from Coober Pedy, it is also the finest quality opalised fossil to have been unearthed to date.

The Museum is grateful to have been awarded more than $180,000 from the Federal Government's National Cultural Heritage Account to acquire the stone, and will be seeking further support to make up the difference.

The Museum is excited to be on threshold of securing for the people of South Australia, and the nation, the most stunning representative of the national gemstone: opal.

In addition to the touring potential of this stunning gallery centrepiece, opals provide our researchers with ongoing clues as to what life roamed in Australia millions of years ago.

Over time, chemicals in ground waters replace bone, shell or wood with minerals such as quartz, opal or calcite. The changes can preserve amazing detail in the specimens, right down to individual cells, and may eventually indicate how long they have been there.

Science has not yet pinpointed the precise conditions for opal formation and how to tell their exact age. Using expertise from our Mineralogy and Palaeontology teams, the South Australian Museum wants to help better explain the stories of these magnificent opalised fossils.

Currently the Museum displays examples of the fossils, including a partially-articulated Addyman Plesiosaur skeleton. The six-metre long skeleton was found in an opal mine at Andamooka in 1983 and is Australia's first cryptoclidid – a species previously known only from the Jurassic period of Europe and the very late Cretaceous period of South America, New Zealand and Antarctica. The Museum also displays opalised shells from an ancient seabed and other fossils.

Visitors can not only marvel at these incredible creatures, but understand the chemical conditions that lead to the "rainbow bones".

Head of Mineralogy Professor Allan Pring wrote a paper with Museum colleague Dr Joël Brugger and University of Adelaide's Benjamath Pewkliang titled Opalisation of Fossil Bone and Wood: Clues to the Formation of Precious Opal.

They took specimens from Andamooka and Coober Pedy, as well as White Cliffs.

Professor Pring says "particular chemical and physical conditions are needed for an opal to form in bone or shell once it lies to rest in sediment, especially the type of fluid in the environment."

Samples were taken and photographed using a diffraction imaging camera. To understand the microstructure of the silica spheres (which reflect opal's bright colours) the scientists placed blocks of the samples in hydrofluoric acid.

Professor Pring says fossils are better preserved if no air is present at the time of opalisation.

"All the necessary minute details of fossils were only preserved when the mineralising fluid moved through the pores in the bone or shell of the structure, and the original molecules were replaced, almost one by one. They dissolved into the fluid and caused opal precipitate at the same time, leaving no gap. The scientists found that opal formation was different depending on where they had collected the specimen from.

The study led the scientists to better understand what caused the opals to form, but they still found it difficult to determine the exact timing of opalisation. As they found that little uranium was present on the specimens, they could not use radiometric dating.

Professor Pring says there are still more questions to answer when looking at the fascinating fossils. "We know that to get opal, the little silica spheres must all grow to the same size. We think that opal originally forms as a jelly and the silica spheres grow slowly over time, settling to the bottom of the gel and solidifying, which is why opalised fossils often show layers of colour."

The South Australian Museum's Opal Fossils Gallery is located on Level 3 in the main building.