Opalised Fossils 


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.

Types of Lightning Ridge Opalised Fossils

Tonnes of opalised plant fossil is extracted from Lightning Ridge opal mines each year. Although some is exquisitely preserved, most is too fragmentary to be informative – other than to show how richly vegetated the area once was.

Diverse pine cones, drupes, stems and seeds are found, sometimes glittering with gem colour. 

100 million years ago, Lightning Ridge was heavily forested with conifers such as Araucarian, podocarp and Kauri pine trees, towering over ferns, seed ferns and ground pines, fungi and lichens, mosses, liverworts and horsetails.

Large pieces of silicified wood are also found in the opal mines; however, these larger pieces are rarely opalised.

Bivalve and gastropod molluscs (mussels and snails) are the most commonly-found opalised fossils at Lightning Ridge. The freshwater species found at Lightning Ridge differ from the molluscs found at White Cliffs and in South Australia, which lived in a marine environment.

Mines that intersect with palaeochannels sometimes contain rich deposits of bivalve molluscs. Occasionally, concentrations of whelks form dense death assemblages in sandstone.

Many different species of opalised mollusc have been found at Lightning Ridge. Although some are relatively common, others are rare.

Freshwater crayfish
The opalised gastroliths of freshwater crayfish are known locally as ‘yabby buttons.’ Crayfish use gastroliths to store calcium from their exoskeletons (‘shells’) before they moult, then release the calcium to harden their new protective coating. Fossil yabby buttons are usually around 10-12mm across.

Fossil shark teeth are rare at Lightning Ridge. Nevertheless, their discovery from time to time suggests that at the time the opal-bearing sediments were laid down, Lightning Ridge wasn’t too far from the shore of the sea that covered much of inland Australia.

At least three species of lungfish lived at Lightning Ridge in the Early Cretaceous period. We know them from the opalised toothplates found in the mines.

Bony fish
A variety of freshwater bony fish from ancient Lightning Ridge have left traces of themselves as opalised fossils, mostly jaw bones and backbones. Relatively few have been recovered, probably because most are so small or fragile that they are not noticed, or are destroyed by the mining machinery.

The oldest frog fossil known in Australia is a tiny opalised upper jaw found in a mine at the Coocoran fields, Lightning Ridge.

Lightning Ridge’s fossils include at least three kinds of land- and swamp-living turtles, including the world’s oldest horned turtle (meiolaniid).

Three different species of crocodile have been identified so far among Lightning Ridge’s opalised fossils. All appear to have been from relatively small crocodiles. The Australian Opal Centre has teeth, back bones and scutes (bony armour from beneath the skin) of these 100-million-year-old crocs.

Plesiosaurs were swimming reptiles that lived during the dinosaur era – the 'reptilian seals' of their time. Some lived in the sea and some in fresh water. Some sea-living plesiosaurs swam upstream to breed.

At Lightning Ridge, opalised plesiosaur teeth are found relatively commonly, but plesiosaur bones are extremely rare. The teeth indicate at least two different kinds of plesiosaur.

Lightning Ridge is blessed with the opalised remains of several kinds of dinosaur: sauropods, theropods, ornithopods, ornithomimosaurids and ankylosaurs. They range in size from the ridiculously enormous to little chicken-sized dinos!

At the Australian Opal Centre we have opalised dinosaur teeth, limb bones, back bones, toe bones, claws and pieces of rib, pelvis and shoulder. Most are in grey, black or amber-coloured potch, but some shimmer with colour. In some opal mines you can even look up to see the underside of dinosaur footprints in the roof.

Pterosaurs were flying reptiles that lived with the dinosaurs, went extinct with the dinosaurs and left no living relatives. Uncrushed pterosaur fossils are quite rare globally, because the bones were hollow and thin-walled. A small number of opalised pterosaur bones have been found at Lightning Ridge.

Snake bones are very delicate and fragile. So far we know of only one opalised snake fossil from Lightning Ridge – a tiny fragment of lower jaw – but as greater care is taken to retrieve tiny fossils from underground, it is likely that further snake fossils will be found.

Just a few tiny bird bones have been found, and perhaps a bird tooth!

Lightning Ridge made the cover of the prestigious journal Nature when its first opalised monotreme mammal was found – the jaw of a platypus-like creature named Steropodon.

Since then, other rare but important monotreme fossils have been discovered, including vertebrae (back bones) that have been donated to the Australian Opal Centre. These are the only opalised monotreme vertebrae in a public collection anywhere in the world.

Sincere thanks to every contributor to the collection of the Australian Opal Centre. Thanks to you, the Centre has the world's most diverse and scientifically significant collection of opalised fossils.

Many items in the Centre’s collection have been donated with the support of the Australian Government's Cultural Gifts Program; our thanks also to the administrators of that program.