Feature Gems & Gemology, Spring 1993, Vol. 29, No. 1

Queensland Boulder Opal

After almost 75 years of inactivity, the deposits of boulder opal in Queensland, Australia, began to attract new interest in the 1960s and 1970s. Queensland boulder opal is found associated with the Winton formation, and results from deposition and dehydration of silica-rich solutions in an iron-rich host rock. At the Cragg mine, one of 69 known opal-mining operations in Queensland in 1991, miners drill as deep as 20 m to search for a deposit and then use open-cut methods to reach the opal-bearing layer identified. In the evaluation of boulder opal, color, pattern, and "composition" are important. Most boulder opal is stable under normal wear, and it is readily separated from its simulants.