In western Queensland, miners have long searched and found rare opal, one unique to the area, the boulder opal.
The boulder opal is found in deposits formed on ironstone, and occurs in weathered sedimentary cretaceous rocks. The weathered rocks form a belt known as the Winton Formation, running a distance of 1000 km. Mining fields sit along the belt of rocks and stretch from Cunnamulla to Kynuna.
The boulder opal is scattered among rocks in these areas, sitting within the concentric layers of ironside rocks and boulders that range in size. Some formations may host a kernel of solid opal or a network of thin veins of opal through the ironstone; this variety of opal is prevalent at Yowah. Pipe opal occurs in sandstone, pipe-like rock formations, which can either be hollow or filled with opal. Different parts of the region also produce different coloured opals, ranging in luminous shades of blue, pink, green, white and black.
When the boulder opal is cut, the ironstone backing is usually left intact, which differs this type of opal from sedimentary precious stones. Boulder opals are fashioned to standard shapes and sizes, and freeform shapes. They are treasured by jewelry lovers and collectors alike for their natural beauty.
