Australian Boulder opal is mainly found in the Northeastern part of Australia in the state of Queensland. Boulder Opals are formed on a host rock called ironstone and can be difficult to mine since the opal gem is not immediately obvious but rather hidden inside thin seams. Once precious opal is identified within seams, miners then “split” the ironstone to reveal larger chunks of beautiful opal faces. This host ironstone rock can range in colour from light brown to deep brown. Like the dark potch of Black Opal, the ironstone backing of Boulder Opals also helps to intensify the vibrancy of colour play. A good piece of Boulder Opal can be more valuable than Black Opal.