The Rainbow Opal - Australia It is an opalised fossil formed as a result of an extraordinary set of natural forces, found uniquely in South Australia. Weighing 72.64 carats, it is a uniform, solid block of intense colour discovered in Coober Pedy in 2003. It is also extraordinarily rare – being an opalised fossil of a belemnite, an ancient squid-like creature over 100 million years old.