FAQ : What are opal fossils? Where are black opal fossils found? What does opalized / opalised mean?
Lightning Ridge – Black Opal Fossils
Only one place on Earth produces black opal fossils – Lightning Ridge in northern New South Wales, Australia. Lightning Ridge is the only opal field in Australia with fossils of diverse land-living organisms – pinecones and platypuses, microscopic protozoans and gigantic dinosaurs. The fossils are usually exact replicas of plant, shell or bone material, and at times they are comprised of gem quality black opal, which is as valuable as diamonds and more beautiful.
Black opal fossils which may be found at Lightning Ridge include remnants of ancient plants, mussels, snails, crustaceans, fish, turtles, plesiosaurs, crocodiles, pterosaurs, dinosaurs, birds, and mammals. 110 million years ago the supercontinent Gondwana was a wilderness of forests of pines, ferns and palms separated by tracts of shallow sea. Dinosaurs and their relatives dominated this landscape, as well as our rare and tiny mammal ancestors. Near the edge of this ancient continent, fragments of the remains of these animals accumulated in the sands of the inland sea. Today deposits at Lightning Ridge in northern New South Wales yield some of the rarest, most beautiful and precious fossils in the world.
Lightning Ridge fossils are three-dimensional replicas of ancient organic objects, transposed into non-precious potch or precious opal. In those that are pseudomorphs, the silica has filled a simple cavity or void, like jelly in a mould, so that only the basic shape and perhaps the surface texture is preserved. However, many specimens are replacement fossils, in which intricate internal structures have been preserved by chemical alteration before the cavity was filled by the silica solution.
Most specimens at the Ridge are a combination of pseudomorph and replacement fossils. Although the transformation to silica has destroyed biomolecular evidence, marrow tissue, blood vessels, capillaries and nerve channels may be perfectly preserved. If the potch is transparent, these features are clearly visible below the surface in opalised bones. A surprising aspect is the opalisation of delicate materials like leaves and even dinosaur skin. Many pieces resemble coprolites, reptilian armour scutes or heavy scales; very occasionally, bone specimens seem to show remnants of tendons or cartilage.
This outstanding quality of preservation is partly because the opal-dirt is extremely fine-grained and an ideal casting medium. Kaolinite, smectite, and illite produce the putty-like properties of the opal clay, the smectite making it plastic and malleable.
Most opal fossils found at the Ridge consist of potch (colourless opal), therefore any fossils with colour are rare and valuable. Many fossils are damaged by machinery during excavation, as pick and shovel based operations are giving way to machine-driven excavations. Removing fossil specimens can be a delicate operation, and colourless fossil specimens are largely ignored by miners searching for colour.
South Australian Fossils
The opal fossils of the South Australian opal fields are both jewels of science and beautiful gems. The Eromanga Sea that covered the interior of Australia 100-120 million years ago was rich in marine life. Ichthyosaurs, plesiosaurs, fish, sharks, ammonites and belemnites swam in the open water. Slow-moving and sedentary animals, such as starfish, crinoids, cockles, mussels, snails and tube-worms lived on the seafloor.
Only those bones and shells that became trapped in seafloor sediment had a chance of becoming fossils. Some were replaced by clear silica, and others by precious opal.
One area which yields a vast resource of opal fossils is Moon Plain, approximately 35km from Coober Pedy in South Australia’s outback. Once part of a vast inland sea which covered most of Australia, Moon Plain once teemed with marine life, but is now a wonderland for palaeontologists. South Australia has the best cold water cretaceous marine deposits in the world, and it is difficult to imagine the extreme freezing temperatures of millions of years ago compared with today’s scorching heat.
Sources :
- “Black Opal Fossils of Lightning Ridge”, Elizabeth & Robert Smith, Kangaroo Press, 1999.
- Origin Energy Fossil Gallery
- Australian Museum Online
