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Oval boulder opal cabochon from Queensland, Australia, measuring 19.6 × 12.9 × 7.2 mm and weighing 23.75 total carats (opal plus matrix).
Piece Details
| Mineral |
Australian Boulder Opal with ironstone matrix |
| Scientific name |
SiO₂·nH₂O in ferruginous sandstone matrix |
| Hardness |
5.5–6 Mohs (opal) / 7 Mohs (quartzitic sandstone) |
| Crystal system |
Amorphous |
| Origin |
Queensland, Australia |
| Dimensions |
19.6 × 12.9 × 7.2 mm |
| Weight |
23.75 carats (opal + matrix) |
| Play of color |
Multi-color: blue, green, yellow, orange |
| Matrix |
Brown-orange ferruginous sandstone |
| Finish |
Mirror polish on the upper face of the opal |
| Treatment |
Untreated |
| Unique piece |
Yes — numbered piece nr.15, unique |
Piece Characteristics
Boulder opal differs from black or white opal due to its inseparable matrix: the opal vein formed within a ferruginous sandstone (ironstone) concretion and cannot be separated from it without destroying the piece.
Specific characteristics of this piece:
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Organic oval shape: preserves the natural shape of the ironstone concretion, without forced geometry
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7.2 mm thickness: provides considerable depth, visible from the side profile
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Irradiating ray pattern: the play of color extends from several simultaneous focal points, creating a network of flashes that covers much of the opal's surface
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Visible matrix on the profile
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23.75 carats: the weight includes matrix and opal. The substantial presence of the piece is notable in jewelry
Jewelry Applications
The size (19.6 × 12.9 mm) and geological appearance of this piece make it a central element for jewelry projects with character:
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Closed bezel setting in 925 silver (recommended): the bezel surrounds the ironstone matrix and the opal together.
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Main pendant: the weight (23.75 ct including matrix) and dimensions make this piece the main element of a pendant.
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Collector's ring: with a robust 925 silver setting for occasional wear.
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Organic wire wrapping: the irregular oval shape allows for wire compositions that follow and reinforce the natural contour of the piece.
The earthy tones of the ironstone matrix combine particularly well with aged or oxidized silver. With polished silver, the contrast between the rock and the metal creates an interesting play of textures.
History and Origin of Australian Boulder Opal
Queensland (Australia) is the producing region for boulder opal. The ironstone nodules containing opal formed in specific sedimentary horizons of the Queensland artesian basin, between 60 and 100 million years ago.
Care Guide for Boulder Opal
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Cleaning: soft, slightly damp microfiber cloth.
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Avoid ultrasonic cleaners: vibrations can propagate through the matrix and generate micro-fractures at the opal-ironstone interface.
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Avoid steam: intense humidity can alter the opal's structure and temporarily darken the matrix.
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Avoid side impacts: the opal-matrix transition zone is the point of greatest structural vulnerability.
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Storage: in an individual padded case, separate from other gems.
Frequently Asked Questions
What is boulder opal and how does it differ from black opal?
Boulder opal has the opal vein inseparably adhered to its ferruginous sandstone (ironstone) matrix. It is mined in Queensland, and the 23.75 ct of this piece includes both the opal and the host rock. Black opal from Lightning Ridge is solid opal without matrix, with the opal's own dark body tone.
Does the 23.75 carat weight include only the opal or also the matrix?
The weight includes both opal and matrix together, as in boulder opal they are inseparable. The pure opal represents the iridescent portion of the piece; the rest corresponds to the ferruginous sandstone. This is standard in the marketing of boulder opal: the carat weight always includes the matrix.
Note: The meanings of crystals are spiritual supports and do not substitute medical prescriptions.
