Geology of Australian Opal

Australian opal is associated with sedimentary rocks of the cretaceous geological error (between 66 and 144 million years old). These rocks are part of the rock formations that are known as the Great Artesian Basin, where vast quantities of water were trapped in sedimentary layers of sandstone deposited by the Shelley Inland Sea (or Ancient Eromanga Sea). Now covered in an impermeable layer of caprock creating an artesian system that traps pressurised water. Some water in the basin is up to 2 million years old! The Great Artesian Basin occurs in a large ‘U’ shape running from the Gulf of Carpentaria down into northern New South Wales and back up into northern South Australia.

Although there is still discussion over the formation of Australian opal, major precious opal producing areas in Australia have similar stratigraphic characteristics. Conducive with the ‘deep weathering’ theory, thick layers of sandstone which have small, localised faults, cracks and joints create the perfect home for opal formation. It is the slow and steady process that occurs when a large body of water above the sandstone has had time to slowly seep down into silica rich sandstone below. Underlining the sandstone is a layer of impermeable clay which has created a water barrier similar to a bowl, allowing silica rich liquid to pool in gaps of the cracks in the sandstone above it. The barrier is known as ‘the level’ by miners because it is in this layer that opal is usually found. In this layer the opal can solidify and form.