Australia is the only place in the world where opalised fossils are found. Fossils are usually formed when plant or animal remains are buried after death, and are slowly encased with sediments and infused with minerals, leaving a rock-like replica of the original organic material. Opalised fossils, however, replace an organism’s organic material with shimmering, solid opal. Excellent detail can be preserved both on the surface and internally, depending on how the opal forms: it’s a hydrous silicon oxide, and begins as a solution of silica in water. If an organism completely rots away, the opal will fill the empty space and create a cast of the external features—but if an organism has left organic material behind, the solution might harden to form a replica of the internal structure too. One of the richest sources of opalised fossils is Lightning Ridge in northwestern New South Wales. The fossils paint a vibrant history of animals who lived there in the Early Cretaceous Period, approximately 110 million years ago—dinosaurs, marine reptiles, fish, early mammals, molluscs, plants… Unfortunately, the opalised fossils are often cut up for jewellery or sold overseas for their beauty and value, but as they’re of significant scientific interest, researchers and paleontologists are working to preserve them.