Boulder Opal
Boulder Opal is hydrated silica in ironstone, SiO₂·nH₂O + Fe-oxide matrix. In the hand, it rates 5.5 to 6.5 (opal); matrix harder-feeling on the Mohs scale, and it runs rainbow opal veins through chocolate ironstone and blue-green flashes.
The geology.
Boulder Opal carries the formula SiO₂·nH₂O + Fe-oxide matrix. It belongs to the amorphous system. It is classified as hydrated silica in ironstone.
It sits at 5.5 to 6.5 (opal); matrix harder-feeling on the Mohs scale. The surface reads vitreous opal in dull matrix. Light behaviour is opaque with translucent opal seams. Specific gravity runs 2.6 to 3.0 (composite). The streak is white (opal); brown (ironstone). It breaks with conchoidal in opal; uneven in matrix.
Chocolate-brown ironstone shot through with thin veins of precious opal, the flash erupts from cracks and seams in the host rock.
You will most often meet it as polished freeforms, cabochons with natural matrix or split 'nuts'.
The origins.
Almost exclusively western Queensland, Australia (Winton, Quilpie, Yowah).
On value, the database notes: mid-range to prized; bright full-spectrum seams climb steeply.
Traditional associations.
In energy work traditions, associated with grounding inspiration into earthy reality.
People buy on this language, so it belongs on the page. It belongs under the facts, though, not over them.
Spotting the real thing.
Chocolate-brown ironstone shot through with thin veins of precious opal, the flash erupts from cracks and seams in the host rock.
Doublets and resin-backed pieces are sold as boulder opal, inspect the opal-matrix junction from the side.
| Looks like | How to tell them apart |
|---|---|
| Opal doublets | Doublets show a flat glue line between opal and backing; boulder opal's opal-to-ironstone boundary is natural and irregular |
| Lab opal in resin | Synthetic mosaics show uniform lizard-skin cells and warm resin matrix |
| Matrix opal (Andamooka) | Treated Andamooka matrix is gray and speckled throughout, not veined ironstone |
At-home tests.
You can run every one of these at a kitchen table. Good light matters more than gear when you are looking at boulder opal.
| Test | What you should see |
|---|---|
| By eye | Natural wavy opal-ironstone boundary (no flat glue plane) |
| By touch | Heavy and cold versus resin composites |
| By eye | Play-of-color patches shift with angle |
Destructive tests belong to a gemologist, not a kitchen table. If a piece matters to you, pay for an appraisal before you take a file to it.
Care & handling.
At 5.5 to 6.5 (opal); matrix harder-feeling on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with conchoidal in opal; uneven in matrix, which is worth knowing before you set it or drop it.
Clean it by hand, dry it properly, and keep it away from ultrasonic cleaners and hot steam.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is SiO₂·nH₂O + Fe-oxide matrix. Crystal system: Amorphous. Specific gravity: 2.6 to 3.0 (composite). Streak: White (opal); brown (ironstone). Cleavage and fracture: Conchoidal in opal; uneven in matrix.
How it reaches the market
Almost exclusively western Queensland, Australia (Winton, Quilpie, Yowah).
What it gets confused with
Opal doublets: Doublets show a flat glue line between opal and backing; boulder opal's opal-to-ironstone boundary is natural and irregular. Lab opal in resin: Synthetic mosaics show uniform lizard-skin cells and warm resin matrix. Matrix opal (Andamooka): Treated Andamooka matrix is gray and speckled throughout, not veined ironstone.
What it is worth
Mid-range to prized; bright full-spectrum seams climb steeply.
Where buyers get caught
Doublets and resin-backed pieces are sold as boulder opal, inspect the opal-matrix junction from the side.
One more thing
Yowah 'nuts' are ironstone concretions that split open like walnuts, some hide a kernel of solid precious opal inside.
An education-first guide.
We do not stock boulder opal. Writing it up regardless is the point of a reference library.
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