Boulder opal is a variety of precious opal that occurs as thin veins or fillings within ironstone boulders. Collectors look for vibrant play-of-color contrasts against the dark, earthy host matrix, which often enhances the brilliance of the opal seams.

Hardness

5.5-6.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

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Boulder Opal

Hardness: 5.5-6.5
Luster: Vitreous
Streak: White
Transparency: Translucent

Spotting boulder opal in the field

Field marks

1
Thin seams of opal running through brown ironstone host rock
2
Play of color sits in a narrow layer rather than through a solid stone
3
The dark ironstone backing makes the color read far more strongly

Boulder opal is Queensland's signature material, from Winton, Quilpie, Yowah and Koroit. The opal formed in cracks and cavities in ironstone concretions, so it is cut with the host rock left on as a natural backing. That backing is not a flaw and not a doublet: it is part of the stone, and a solid piece of boulder opal is worth more than an assembled stone that looks similar. Yowah nuts are the small ironstone nodules with opal centers from the same country.

Straight answers

Is it worth cutting?: Follow the seam rather than imposing a shape on it. Free-form cutting is the norm precisely because the opal layer dictates what is possible.
What is it really worth?: Moderate to high, and generally more stable than Virgin Valley material, which is part of why it is popular. Solid natural boulder opal outsells assembled stones considerably.
Can you legally keep it?: Australian material, mostly on mining leases. It reaches collectors through the trade rather than through public digging.

Often found alongside boulder opal

Minerals reported to co-occur with boulder opal. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: SiO₂·nH₂O
Mohs hardness: 5.5-6.5
Density: 2.1-2.3 g/cm³
Colors: Black White Gray Brown Blue Green Red Yellow
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Amorphous
Crystal habit: Veins and Patches in Host Ironstone
Cleavage: None
Rarity: Uncommon
Uses: Gemstone, Lapidary, Collector
Host rock: Sedimentary Ironstone Concretions
Typical price: $20-100 for specimens, $200-5000+ for high-quality finished gems

Where rockhounds find boulder opal

Classic worldwide localities

Queensland, Australia
Winton, Australia
Yowah, Australia

Field-hunting tip

Look in sedimentary ironstone concretions country, the host setting where boulder opal typically forms. If you start seeing ironstone, limonite, sandstone in float, you are in the right ground. Field specimens usually show a veins and patches in host ironstone habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify boulder opal?+

Mohs hardness is 5.5-6.5. It typically shows a vitreous luster. The streak is white. Common colors include black, white, gray, brown.

Where is boulder opal found?+

Notable localities include Queensland, Australia; Winton, Australia; Yowah, Australia.

How much is boulder opal worth?+

Typical asking prices fall in the range of $20-100 for specimens, $200-5000+ for high-quality finished gems. Quality, size, and provenance can move individual specimens well outside that range.

What minerals are found with boulder opal?+

Boulder Opal commonly co-occurs with Ironstone, Limonite, Sandstone. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does boulder opal form in?+

Boulder Opal typically forms in sedimentary ironstone concretions. Working float back to the host body is the standard way to chase a fresh occurrence.

What is boulder opal used for?+

Boulder Opal is used in gemstone, lapidary, collector.

Find boulder opal on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.