Boulder Opal

Physical Properties

Hardness: 5.5-6.5 (opal) / 6-7 (ironstone); Color: Brown host rock with flashes of green, blue, and red; Lustre: Sub-vitreous to waxy; Crystal structure: Amorphous; Fracture: Conchoidal; Specific Gravity: 2.1-2.5.

Formation & Geological History

Formed from silica-rich water seeping into cracks and voids within ironstone boulders (sedimentary rocks). Most Australian boulder opals formed during the Cretaceous period, roughly 100 million years ago.

Uses & Applications

Primarily used in fine jewelry such as rings and pendants, and as high-value collectors' specimens. Due to the ironstone backing, it is more durable than other opal types.

Geological Facts

Boulder opal is the second most valuable type of opal after Black Opal. It is unique because it is found and cut with the natural host rock on the back, which provides a dark base to enhance the play-of-color.

Field Identification & Locations

Identify by looking for thin veins of iridescent color embedded within a brown, heavy ironstone matrix. It is almost exclusively found in the Queensland opal fields of Australia (e.g., Winton, Quilpie).

Identified on: 4/15/2026

Mode: Standard