gemstone

Boulder Opal (Koroit Matrix Opal Egg)

Hydrated Amorphous Silica (SiO2·nH2O) in Ironstone Matrix

AI-generated identification · may be incorrect

Hardness 5.5 - 6.5 on Mohs scale; vitreous to resinous luster; amorphous crystal system; displays vibrant iridescent play-of-color (flashes of blue, purple, orange, and green) enclosed within a reddish-brown, swirling ferruginous sandstone or ironstone matrix;…

Identified October 3, 2026More gemstone →

Explore Boulder Opal (Koroit Matrix Opal Egg) in the encyclopedia →

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Physical properties

Formation & geological history

Formed during the Cretaceous period (roughly 100 to 65 million years ago) when deep weathering and groundwater dissolved silica from overlying rocks, trickling into voids, fractures, and concretionary cavities within sedimentary ironstone boulders.

Uses & applications

Prized in lapidary work, jewelry making, and ornamental carvings. Polished into cabochons, freeform carvings, or collector display shapes such as eggs and spheres.

Geological facts

Boulder opal is unique to Queensland, Australia, and is one of the sturdiest varieties of natural opal because the precious gem material naturally adheres to the dense, iron-rich host rock.

Field identification & locations

Identifiable by its distinctive pattern of vibrant, fire-bearing silica patches distributed throughout a swirling, earthy brown-red ironstone matrix. Prime localities include the Koroit, Yowah, and Winton opal fields in southwestern Queensland, Australia.