Triangular Australian boulder opal cabochon, measuring 18.9 × 11 × 7.25 mm and weighing a total of 15.1 carats (opal plus ironstone matrix). The triangular shape is uncommon in boulder opal: it requires extracting the ironstone nodule with the correct orientation and cutting while respecting the angles without fracturing the opal vein. The multicolor play of color—blue, green, and red flashes—radiates from different points on the surface with high intensity. Unique piece numbered nr.16.
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 |
Australia |
| Dimensions |
18.9 × 11 × 7.25 mm |
| Weight |
15.1 carats (opal + matrix) |
| Shape |
Triangular |
| Play of color |
High intensity multicolor: blue, green, red |
| Matrix |
Dark Ironstone (ferruginous sandstone) |
| Finish |
Mirror polish on the opal face |
| Treatment |
Untreated |
| Unique piece |
Yes — numbered piece nr.16, unrepeatable |
Characteristics of the piece
Boulder opal is a variety of opal in which the opal vein and the ironstone (ferruginous sandstone) matrix are mineralogically inseparable. Unlike solid black opal, boulder opal includes the host rock as an integral part of the piece. In this case, the dark matrix intensifies the contrast with the opal's play of color, producing a visual effect similar to black opal but with the characteristic texture and added weight of the host rock.
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Triangular shape: unusual geometry for boulder opal. The three angular vertices require specific protection during setting.
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Dark matrix: the ironstone in this piece has a very dark tone that maximizes contrast with the play of color.
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High chromatic intensity: the play of color covers a large part of the visible opal surface with highly saturated flashes.
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7.25 mm thickness: provides significant presence and optical depth for the piece's dimensions.
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15.1 carats: total weight including opal and matrix. Proportional to the 18.9 mm triangular size.
Jewelry Applications
The triangular shape (18.9 × 11 mm) and the visual impact of the play of color make this piece ideal for designer jewelry projects in 925 silver:
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Bezel setting in 925 silver (recommended): the bezel surrounds the three vertices of the triangle, distributing setting pressure evenly. The organic irregularity of the boulder's outline requires manual adjustment of the bezel to the piece's actual contour.
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Statement pendant: the dimensions and triangular geometry create a central pendant with great visual impact on a 45–60 cm silver chain.
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Artistic occasional-wear ring: with a robust 925 silver mounting that protects the three vertices. The 7.25 mm thickness requires a medium-high profile mounting.
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Wire wrapping in 925 silver: feasible with weaves that do not exert punctual pressure on the vertices. Triangular compositions in wire wrap are visually very effective.
The dark ironstone matrix combines particularly well with oxidized or blackened silver. With mirror-polished silver, the contrast between the shiny metal and the matte texture of the rock is equally attractive.
History and Origin of Australian Boulder Opal
Australian boulder opal is mainly extracted from deposits in the outback. The ironstone nodules containing opal formed between 60 and 100 million years ago when silica-rich water penetrated cracks and cavities in the ferruginous sandstone. As it slowly evaporated, the silica deposited layers of nanospheres with a periodic structure that diffract visible light, producing the play of colors. Each nodule is unique in shape, size, opal distribution, and play of color characteristics, making each boulder cabochon an unrepeatable piece.
Boulder Opal Care Guide
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Cleaning: soft, slightly damp microfiber cloth. The ironstone matrix is porous; avoid prolonged immersion in water.
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Avoid ultrasonics: vibrations can cause microfractures at the opal-ironstone interface and within the opal.
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Avoid steam: intense humidity can alter the internal structure of the opal and temporarily darken the matrix.
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Avoid impacts on vertices: the three angles of the triangle are the most vulnerable points. With a hardness of 5.5–6 Mohs, opal is susceptible to chipping in angular areas.
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Storage: in an individual padded case, separate from other harder gems.
Frequently Asked Questions
How does this boulder opal differ from black opal?
Solid black opal has no matrix: it is pure opal with its own dark base. Boulder opal inseparably carries its ironstone host rock, which in this piece is very dark and creates a visual effect similar to black opal but with the structure and additional weight of the sandstone matrix. The difference is mineralogical: one is pure opal, the other is opal attached to its host rock.
Does the 15.1 carat weight include only the opal or also the matrix?
In boulder opal, the weight always includes both opal and matrix, as they are inseparable. The total weight of this piece (15.1 ct) corresponds to the ironstone nodule with the opal vein included.
Is it feasible to set a triangular shape in silver?
Yes. A bezel setting in 925 silver is the most suitable method for angular shapes like this triangle. A jeweler experienced with soft stones and irregular shapes can manually adjust the bezel to the exact contour of the piece. The result is a mounting that protects the three vertices and allows the maximum surface of the opal to be seen.
Note: The meanings of crystals are spiritual supports and do not replace medical prescriptions.
