At the heart of the Australian Opal Centre is a magnificent collection of 110-million-year-old fossils, from a period of time called the Early Cretaceous. It was a time when dinosaurs and other ancient creatures lived in the place we now know as Lightning Ridge, and when great marine reptiles swam in a shallow sea over much ofÂ inland Australia, where the opal mining towns of White Cliffs, Coober Pedy, Andamooka, Mintabie and Lambina now lie.
These are no ordinary fossils (if there is such a thing): these incredible relics are made of solid opal, sometimes with rainbows of shimmering colour. Australia is the only place on Earth where opalised animal fossils are found. These fossils are of global scientific interest and are among the most beautiful and valuable in the world.
Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried. We get opalised fossils of two kinds:
i. Internal details not preserved: Opal starts as a solution of silica in water. If the silica solution fills an empty space left by a shell, bone etc that has rotted away – like jelly poured into a mould – it may harden to form an opalised cast of the original object. Most opalised shell fossils areÂ ‘jelly mould’ fossils – the outside shape is beautifully preserved, but the opal inside doesn’t record any of the creature’s internal structure.
ii. Internal details preserved: If the buried organic material hasn’t rotted away and a silica solution soaks into it, when the silica hardens it may form an opal replica of the internal structure of the object. This happens sometimes with wood or bone.
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.
A large diversity of exquisite pine cones, drupes, stems and seeds are found, sometimes glittering with gem colour. Those that have not been through mining machinery retain remarkable detail.
110 million years ago, Lightning Ridge was heavily forested with primitive conifers such as Araucarian, podocarp and Kauri pines, which towered over ferns, seed ferns, ground pines, fungi and lichens, mosses, liverworts and horsetails.
Large pieces of silicified wood are found in the opal mines; however, these larger pieces are rarely opalised.
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.
Lightning Ridge’s fossils include at least three kinds of land- and swamp-living turtles, including the world’s oldest horned turtle (meiolaniid).
The teeth indicate at least a couple of different kinds of plesiosaur.
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 the roofs of some opal mines you can even look up to see the underside of dinosaur footprints.
Sincere thanks to everyone who has contributed to the collection of the Australian Opal Centre. Many items in the Centre’s collection have been donated with the support of the Cultural Gifts Program of the Australian Federal Government; our thanks also to the administrators of that program.