Boulder Opal

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Known for its lively, flaming color displays, boulder opal is in high demand by collectors and jewelry designers. As opposed to cut opal gems, boulder opal features small opal veins running through its matrix. The precious opal's rainbow of colors make a dramatic contrast to the rich earthy reds and browns of the host rock, creating a one-of-a-kind display.

Boulder Opal Polished
Boulder Opal Classification
Common Name Boulder Opal
Species Opal
Boulder Opal Optical Properties
Transparency Transparent - Opaque
Dispersion Strength: None
Refractive Index 1.450
Optic Character NA
Optic Sign NA
Polariscope Reaction Singly Refractive (SR) With ADR, Aggregate (AGG)
Fluorescence SWUV: Variable
LWUV: Variable
Pleochroism None
Boulder Opal Characteristic Physical Properties
Hardness 5-6.5
Specific Gravity 1.980-2.250
Toughness Varies
Inclusions Boulder opal will have seams of precious opal in brown ironstone, quartzite, rhyolite or basalt host rock.
Luster Vitreous, Resinous
Fracture Conchoidal, Uneven
Cleavage None
Boulder Opal Chemistry & Crystallography
Chemical Name hydrated silicon dioxide
Chemical Formula SiO2-nH2O
Crystal System NA
Chemistry Classification Silicate

Boulder Opal Colors

  • Brown Boulder Opal Brown
  • Multi-color Boulder Opal Multi-color

Alternate Names

Precious Opal, Natural Opal Type 2, Yowah Opal

Countries of Origin

Mexico; Australia


Boulder opals are small opal veins running through matrix, the host rock. The precious opal's myriad of colors creates a breathtaking display in contrast to the rich earthy reds and browns of the host rock. The result is a one-of-a-kind stone that is sought after by gemstone and jewelry lovers for the promise of a rainbow of beauty. Don't let its beauty fool you: opal is one tough stone, it's quite hard (5 - 6 ½ on the Mohs scale) and wonderfully wearable.

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More About Boulder Opal

We have evidence that opals have been around since at least 200 B.C., so they have plenty of folklore that has grown around their beauty. Some we like include the Arabian belief that opals fall from heaven in flashes of lightning; the Oriental belief that opal was the stone of hope; and from the ancient Greeks, the belief that it bestowed its owners with the gift of prophecy.

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Yowah Nut

Yowah Nut is a variety of Boulder Opal that derives its name from a locality in the Australian Outback. The nodules are about walnut size and will have colorful veins running throughout.

Yowah Nut Boulder Opal
Yowah Nut Classification
Common Name Yowah Nut
Yowah Nut Characteristic Physical properties
Toughness Good
Inclusions Precious opal in ironstone or sandstone matrix
Tim Matthews


Tim Matthews

Tim Matthews is President and Chief Executive Officer of Jewelry Television® (JTV), as well as a member of the company's Board of Directors. He oversees and leads all aspects of the company's powerful omni-digital retail platform that uniquely specializes in fine jewelry and gemstones. His passion for business and gemstones has led him to become a recognized expert in the field of gemology. He is a life member of the Gemmological Association of Great Britain (Gem-A) and has earned Gem-A's highest degrees, the Gemmology (FGA) and Diamond (DGA) diplomas. He is also a Graduate Gemologist (GG) of the Gemological Institute of America (GIA) and has also completed GIA's Graduate Diplomas in Diamonds, Colored Stones and Pearls. Under his leadership, JTV has become the leader in the sourcing and selling of color gemstones and jewelry.

This page was created on June 27, 2014.

This page was last edited on October 24, 2019.