What appears to be the start of a great sandy desert is, in fact, the result of human activity over the better half of 100 years.
Stretching for hundreds of kilometres, you can find spill piles from people searching for opal. The reason it looks like sand is because opals are essentially hydrothermally altered silica. Therefore the spill piles are the silica-rich white sand and mudstones that have not been altered. Opals can be found in a few places in Australia, however, one of the larger fields include Cooper Peedy, and others include Lightning Ridge in NSW.