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A free-form Australian Queensland boulder opal cabochon, measuring 10 × 6.5 × 5 mm and totaling 3.4 carats (opal plus ironstone matrix). Small in size but with its own personality: the multicolored flashes of the opal emerge from the earthy ironstone matrix in a compact format ideal for earrings, small pendants, or minimalist jewelry pieces with geological character.
Item Specifications
| 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 |
10 × 6.5 × 5 mm (free form) |
| Weight |
3.4 carats (opal + matrix) |
| Play-of-color |
Multicolor emerging from earthy matrix |
| Matrix |
Brown ironstone (ferruginous sandstone) |
| Finish |
Polished in the opal area |
| Treatment |
Untreated |
| Unique Piece |
Yes — numbered piece no.9, one of a kind |
Item Characteristics
Despite its compact dimensions (10 × 6.5 mm), this piece condenses all the characteristics of Australian boulder opal into a manageable format for small jewelry pieces. The 5 mm thickness is remarkable for the cabochon's dimensions, indicating that the opal vein has good depth within the matrix.
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Organic Free Form: the outline follows the natural shape of the ironstone nodule, without geometric adjustment. Each free-form boulder piece is completely different from any other.
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Compactness: the maximum length of 10 mm allows for use in paired earrings, small pendants, or as a secondary accent in larger compositions.
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Opal-Matrix Contrast: the transition between the iridescent opal area and the matte, earthy texture of the ironstone is visible from the side profile.
Jewelry Applications
The small format (10 × 6.5 mm) of this piece is suitable for projects that require smaller-scale gems:
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Individual Earrings: a single piece in a minimalist 925 silver earring with a closed bezel setting. The asymmetrical free form adds character.
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Non-Identical Paired Earrings: combined with another boulder piece of similar dimensions, it creates earrings where each piece is distinct but the material unifies the set.
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Small-Scale Pendant: ideal for delicate pendants on a thin chain, where the small scale of the opal contrasts with the delicacy of the metal.
Care Guide for Boulder Opal
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Cleaning: soft, slightly damp microfiber cloth. Avoid prolonged immersion in water.
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Avoid Ultrasonics: they can generate microfractures at the opal-ironstone interface.
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Avoid Steam: intense humidity can alter the internal structure of the opal.
History and Origin
Queensland boulder opal is primarily mined in the outback region, in deposits such as Yowah, Quilpie, and Koroit. The formation process began 60–100 million years ago when silica-rich water penetrated cracks in sedimentary ironstone nodules. As it slowly evaporated, the silica formed layers of nanospheres with a periodic structure that diffracts visible light, producing the characteristic play-of-color.
Frequently Asked Questions
Does the boulder opal in this piece have visible play-of-color?
Yes. The multicolored flashes are visible in the opal area of the piece, which emerges from the earthy ironstone matrix.
Is it feasible to use this piece in everyday earrings?
With the usual precautions for opal (avoiding knocks, sudden temperature changes, and exposure to chemicals), this piece is suitable for regular wear earrings. The total weight of 3.4 ct is light for earrings.
Is the ironstone matrix an integral part of the piece or can it be removed?
In boulder opal, the opal vein and the ironstone matrix are inseparable: they are mineralogically adhered. Attempting to separate the opal from the matrix would destroy the piece. The matrix is a defining characteristic of boulder opal, not a defect.
Note: The meanings of crystals are spiritual supports and do not replace medical prescriptions.
