Koroit opal field, Boulder opal mining area in Queensland, Australia

Koroit opal field is a mining area in Queensland where opals form naturally within ironstone boulders found in sedimentary rock layers. The gems sit embedded in these rock formations and are extracted using both surface digging and underground shafts.

Lawrence Rostron found opal at Koroit in 1897, and a small venture began digging. The Scottish and Australian Opal Mines took over and started mining in May 1903, sinking 150 shafts and driving 460 metres of tunnels for matrix and boulder opal. Falling prices, caused by a glut from White Cliffs, stopped work. In 1907 a single worker found the 15 cm 'Great Australian' opal; further attempts in 1925 failed, and mining revived in the 1970s.

The mining site attracts professional miners and recreational fossickers who seek valuable gemstones through open-cut mining and underground shaft methods.

This location sits about 80 kilometers northwest of Cunnamulla in a remote setting without electricity or running water. Visitors should come prepared for basic conditions and bring plenty of water and supplies.

In 1907 a single worker at Koroit found a rounded opal about 15 cm across, one of the largest ever recovered; it was named the Great Australian. Opal here occurs as a kernel inside small concretions within the Winton Formation, a sedimentary belt some 300 km wide.