Boulder opal is precious opal that formed in thin veins and pockets inside ironstone boulders. Cutters leave the opal attached to its host rock, so each stone is a slice of gem and earthy brown ironstone together. That dark natural backing makes the play-of-colour glow vividly, a bit like a built-in black opal effect. It comes almost entirely from western Queensland in Australia, from opal fields around Winton and Quilpie. Every piece is cut freeform to follow the vein, so no two are alike.
The opal layer sits around 5.5 to 6.5 on the Mohs scale, but the ironstone backing gives boulder opal more stability than most solid opals. Even so, treat it kindly: no ultrasonic cleaners, no harsh chemicals, and avoid sudden heat. It is cut as cabochons and freeform shapes for pendants, earrings and occasional-wear rings. Because the dark backing is natural rock, boulder opal gives you doublet-style drama in a fully natural, untreated stone. Just wipe it clean with a soft, damp cloth.
Other varieties in the opal family.
