Oval Australian Queensland boulder opal cabochon, measuring 19.6 × 12.9 × 7.2 mm and totaling 23.75 carats (opal plus matrix). The iridescent opal vein emerges from the brown-orange ferruginous sandstone matrix: multi-colored rays radiate from multiple points, creating a spectacular contrast between the chromatic vibrancy of the opal and the earthy solidity of the host rock. Unique piece numbered nr. 15.
Piece Data Sheet
| 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 |
Multicolor: 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, unrepeatable |
Piece Characteristics
Boulder opal is distinguished from black or white opal by its inseparable matrix: the opal vein formed within a concretion of ferruginous sandstone (ironstone) and cannot be separated from it without destroying the piece. This is a geological characteristic, not a defect: the matrix provides additional structural stability to the opal and creates a visual contrast between the vitality of the play of color and the solidity of the host rock.
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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Thickness of 7.2 mm: provides considerable depth, visible from the side profile
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Irradiating ray pattern: the play of color extends from several simultaneous foci, creating a network of flashes that covers a large part of the opal's surface
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Visible matrix in profile: the oval cut leaves the transition between sandstone and opal visible on the sides of the piece
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23.75 carats: 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. The organic irregularity of the contour requires manual adjustment of the bezel, a common technique in free-form cabochon jewelry making.
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Main pendant: the weight (23.75 ct including matrix) and dimensions make this piece the main element of a pendant. The chain should be proportional to the weight.
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Collector's ring: with a robust 925 silver setting for occasional wear. The 7.2 mm thickness requires a high-profile setting.
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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 antique 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. Silica precipitated into cracks and cavities in the ferruginous sandstone, creating opal veinlets that over time developed the phenomenon of play of color. Each nodule is unique: its shape, size, position of the opal vein, and play of color are unrepeatable, making each cabochon a collector's item.
Boulder Opal Care Guide
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Cleaning: soft, slightly damp microfiber cloth. Sandstone matrix is more porous than opal; avoid prolonged immersion in water.
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Avoid ultrasonics: vibrations can propagate through the matrix and generate microfractures 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 an opal vein inseparably attached 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 a matrix, with a dark base characteristic of opal. Boulder has a more variable base (it can be dark due to the matrix, or with variations in earthy tones).
Does the 23.75 carat weight include only the opal or also the matrix?
The weight includes opal and matrix together, as in boulder opal, both are inseparable. The pure opal represents the iridescent portion of the piece; the rest corresponds to the ferruginous sandstone. This is standard in the commercialization of boulder opal: the carat weight always includes the matrix.
Can I set this piece in a ring for daily wear?
The hardness of opal is 5.5–6 Mohs, lower than that of quartz. For daily wear in a ring, a high closed bezel setting and selective use are recommended (avoid intense manual activities while wearing the ring). For occasional use or occasional jewelry, the piece is perfectly suitable with the appropriate setting.
Note: The meanings of crystals are spiritual supports, they do not replace medical prescriptions.
