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

gemstoneuncommonOpal family

Boulder Opal

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

Is this boulder opal?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch boulder opal with a known reference. Boulder Opal sits at Mohs 5.5-6.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Boulder Opal leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Boulder Opal typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: black, white, gray, brown, blue, green, red, yellow.
5
Look at form & habit
Crystal system: amorphous. Typical habit: veins and patches in host ironstone.

Often confused with

Boulder Opal vs. its common look-alikes — and how to tell them apart in the field.

How to tell apart: Luster reads vitreous on Boulder Opal and waxy on Matrix Opal.

Compare →

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 — that is 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 rocks look like boulder opal?+

Boulder Opal is most often confused with Opalite, Matrix Opal. A quick hardness test and a streak check separate the look-alikes faster than color alone.

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.