Opal Fossils of Lightning Ridge

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Opal Fossils of Lightning Ridge

Excerpt of an article from the Australian Opal Centre

Opal Fossils Lightning Ridge Australia


At the heart of the Australian Opal Centre is a magnificent collection of 100-million-year-old fossils, from the Early Cretaceous period. It was a time when dinosaurs and other ancient creatures lived where Lightning Ridge now stands, and when ancient reptiles swam in a shallow sea covering much of inland Australia, including where the opal fields of White Cliffs, Coober Pedy, Andamooka, Mintabie and Lambina are now.

These are no ordinary fossils (if there is such a thing): these incredible relics are made of solid opal. They are Australian National Treasures, of global scientific interest, and among the most beautiful and valuable fossils in the world.

How do opalised fossils form?
Opal forms in cavities within rocks. If the cavity is there because part of a living thing – for example a bone, shell or pinecone – was buried in the sand or clay before it turned to stone, then the opal can form a fossil replica of the object that was buried.

A fossil is simply “the remains or traces of an ancient animal or plant preserved in rock”. Opalised fossils form in ways similar to other fossils, except that here they are preserved in silica. Elsewhere, fossils are preserved in minerals such as agate, pyrite or limestone.

The sediments that buried plant and animal remains in the opal fields were rich in silica from ancient volcanoes, so here we have fossils preserved as silica in the form of opal.

Opalisation of plants and animal remains happens in two ways, and at Lightning Ridge, a combination of the following two processes is seen in many specimens.

  • Internal details not preserved (‘jelly mould’ fossils). Opal starts as silica dissolved in water. When the silica solution fills an empty cavity left by a shell or bone that has rotted away –  like jelly poured in a mould – it may harden to form an opalised cast of the original object. In these fossils, outside features can be beautifully preserved, but the internal structures are not recorded.
  • Internal details preserved. If the silica seeps into the organic material before it decomposes, then the organic molecules can be replaced by silica. This preserves very fine details of structures inside the bone or plant. When the silica is transparent, this internal anatomy is visible from the outside: the fossil is ‘see through’.

Opalised Fossils of Lightning Ridge

What is important about Lightning Ridge’s opalised fossils?

  • They can be incredibly beautiful!
  • They are providing new and fascinating information about Australia’s ancient heritage and the evolution of plants, animals and environments on the Australian continent.
  • Of all the Australian opal fields, Lightning Ridge and some boulder opal fields are the only places that have opalised fossils of land-living and freshwater plants and animals. The other Australian opal fields have fossils of saltwater or marine organisms, which provide other important information about Australia’s past and the ancient Eromanga Sea.
  • Australia is the only place on earth that produces opalised bones of land-living animals including dinosaurs – and most of these are from Lightning Ridge.
  • Lightning Ridge is the only significant dinosaur site in New South Wales. Opalised bones from fields like Coober Pedy, Andamooka and White Cliffs are from plesiosaurs, ichthyosaurs and pliosaurs, which are marine reptiles, not dinosaurs.
  • Fossils are preserved here in silica in the form of opal. Some of them are see-through, including the only transparent fossils of large animal bones in the world.
  • The Lightning Ridge fossils are of plants and animals that lived close to the South Pole, in global greenhouse conditions, in habitats unknown anywhere today.  They are a window onto Australia’s past – important for scientists who study the evolution of Australian plants, or dinosaurs, or mammals, or climate change, geology and many other aspects of earth history.
  • Fossil formation is closely linked to opal formation. Study of opalised fossils could provide important new information on opal formation, and help with opal exploration and prospecting.

Read the full article on the Australian Opal Centre website.


Another article from The Smithsonian magazine :

Opal is found commonly around the world, but precious opal is very rare and geologists say nothing compares to that produced in Central Australia. Opalized fossils have likewise been found at other opal fields in Australia, but Lightning Ridge stands out for preserving the greatest diversity of extinct freshwater and land-living creatures, including dinosaurs galore. A hundred million years ago, the now-arid interior of Australia was flooded by a vast inland sea, and Lightning Ridge sat on its edge. As the sea retreated, the leading theory goes, it exposed a peculiar mix of sediments forming a sandstone laced with reactive minerals. Rocks nearer the surface began to weather, producing a groundwater rich in silica. It lay in cracks in the rock and filled any cavities, including the skeletal remains of dinosaurs and other long-extinct creatures.
Read more here


From the Australian Museum:

Why is Lightning Ridge important?

Deposits at Lightning Ridge yield some of the rarest, most beautiful and precious fossils in the world. The sandstone at Lightning Ridge once formed the floor of an ancient shallow inland sea where plants, aquatic life and occasionally the bones and teeth of animals were preserved. As they tunnel through these sediments searching for precious opal, miners sometimes find these fossils.

The most famous and significant fossils from Lightning Ridge are those of some early mammals. Mammal fossils are not often found in Cretaceous fossil deposits, since the generally rare, tiny and delicate mammals of this period were far outnumbered by the more successful and diverse dinosaurs. In Australia Cretaceous mammal fossils are almost unknown, which is why the Lightning Ridge fossils are so important.

Palaeontologists searching for fossils of dinosaurs and primitive mammals at Lightning Ridge consult with opal miners to see what they have found, or obtain permission to sift through spoil heaps and excavate in opal mines. Without the help of miners, many fossils, including Steropodon and Kollikodon, would never have been recognised. As well as being beautiful gems, the rare and often valuable fossils from Lightning Ridge have the potential to solve many mysteries about our ancient mammalian ancestors.

Read more here