Rare Boulder Opal

This one-of-a-kind, museum-grade boulder opal displays striking bands of emerald green and aqua blue, vividly contrasted against its natural ironstone matrix. The patterning creates an organic, layered effect that mimics natural concentric forms—though not growth rings in the biological sense.

What Is Boulder Opal?

Boulder opal is a type of precious opal that forms within cracks, cavities, and seams in ironstone boulders, primarily in Queensland, Australia. Unlike solid opals, it remains bonded to its host rock, which enhances both visual contrast and durability.

Why It Displays Color

The vivid "play-of-color" in precious opal—including boulder opal—arises from light diffraction caused by the ordered arrangement of silica spheres. The size and spacing of these colloidal spheres determine the visible color spectrum:

• Red/orange flashes = larger spheres

• Green/blue flashes = smaller spheres

Not all opal shows color. Common opal lacks this internal structure and appears milky or waxy with no iridescence.