Brick Pattern Opal

A structured play-of-colour variety prized for its geometric regularity

Gem varietiesView in dictionary · 1,210 words

Brick pattern opal is a variety of precious opal distinguished by a play-of-colour in which discrete rectangular or square patches of spectral colour are arranged in a regular, staggered grid closely resembling the coursed layout of a brick wall. The pattern is produced by the internal microstructure of the opal itself — specifically, by the ordered packing of amorphous silica spheres into small, well-defined domains — and is considered one of the more desirable and less commonly encountered pattern types in the opal trade. It is most celebrated in Australian black opal, where a dark body tone provides the contrast necessary to display the geometric colour blocks at their most vivid.

The Physics of Pattern Formation

All precious opal owes its play-of-colour (opalescence in the strict gemmological sense) to the diffraction and interference of visible light by a three-dimensional lattice of silica spheres, typically 150–400 nanometres in diameter, suspended in a silica matrix. When the spheres are uniform in size and closely packed in an orderly, face-centred cubic or hexagonal arrangement, they act as a diffraction grating, selectively reinforcing wavelengths of light that correspond to the sphere diameter and the angle of observation.

The specific pattern visible at the surface — whether pinfire, harlequin, rolling flash, or brick — is governed by the size and lateral extent of the coherent domains within this sphere lattice. In brick pattern opal, the coherent domains are elongated or rectangular in cross-section and are arranged in a staggered sequence analogous to the offset courses of a masonry wall. Each domain diffracts a single dominant colour or a narrow colour range; adjacent domains, offset by approximately half a domain-width, produce the characteristic alternating, stepped arrangement of colour patches. The regularity of the pattern implies an unusually consistent sphere size and packing geometry sustained over comparatively large lateral distances within the stone, which is precisely why the pattern is uncommon.

The play-of-colour is most vivid and the pattern most legible when the stone is viewed perpendicular to the plane in which the colour layers lie — that is, face-on to the colour bar. Oblique viewing angles cause the diffracted wavelengths to shift toward the blue end of the spectrum and can partially dissolve the geometric definition of the pattern.

Distinction from Related Patterns

Gemmological literature recognises a hierarchy of opal patterns based on their rarity and visual impact. The principal named patterns include:

Pinfire (pinpoint): Very small, closely spaced points of colour; the most frequently encountered pattern.
Flash: Large, sweeping sheets of a single colour that roll across the stone as the viewing angle changes.
Rolling flash: A broad wave of colour that appears to move across the stone.
Harlequin: Large, roughly equidimensional patches of colour in a mosaic arrangement; widely regarded as the rarest and most valuable pattern.
Brick: Rectangular patches in a staggered grid; rarer than pinfire or flash, less common than harlequin in the finest grades, though the boundary between brick and harlequin can be contested when patches approach squareness.
Peacock: Predominantly blue and green colour play, not strictly a geometric pattern designation.
Chinese writing (ribbon): Irregular, script-like streaks of colour.

The distinction between brick and harlequin is primarily one of patch geometry: harlequin patches are conventionally described as roughly square or equidimensional and are arranged in a more random mosaic, whereas brick patches are distinctly rectangular and staggered. In practice, cutters and dealers sometimes apply the terms loosely, and a stone exhibiting large, nearly square patches in a staggered arrangement may be described as either brick or harlequin depending on the vendor's interpretation. The Gemmological Association of Australia and major opal auction catalogues generally reserve harlequin for the most regular, equidimensional mosaic and use brick for clearly elongated, coursed patches.

Principal Sources

Australian opal fields produce the overwhelming majority of gem-quality brick pattern material. Lightning Ridge in New South Wales is the pre-eminent source of black opal with well-defined brick pattern; the dark potch (common opal) backing characteristic of Lightning Ridge stones provides the body tone that maximises colour contrast. Coober Pedy in South Australia and Mintabie, also in South Australia, yield predominantly white or crystal opal, in which brick pattern can occur but is less visually dramatic owing to the lighter body tone. Andamooka, South Australia, occasionally produces brick pattern material in its semi-black and crystal opal.

Ethiopian opal from the Welo (Wollo) Province has attracted significant trade attention since its emergence in the early 2010s. Welo opals can display brick pattern, though the hydrophane nature of many Ethiopian stones — their capacity to absorb water and temporarily alter their appearance — introduces additional considerations for grading and care. Boulder opal from Queensland, in which precious opal occurs in thin seams within ironstone host rock, can also exhibit brick pattern, though the irregular geometry of the seams often interrupts the pattern's regularity.

Valuation Factors

Within the opal trade, brick pattern contributes positively to value, though the assessment of any opal involves multiple simultaneous criteria. The principal factors bearing on the value of a brick pattern opal are:

Body tone: A dark or black body tone (N1–N4 on the standard Australian opal body-tone scale) intensifies the apparent brightness of the colour play. Black opal with brick pattern commands a substantial premium over equivalent crystal or white opal.
Colour range and brightness: Stones displaying the full spectral range — particularly those showing red, which requires the largest sphere diameter and is statistically rarer — are valued most highly. Brightness (the intensity of the colour play under standard illumination) is assessed on a scale of 1 to 5 by the Gemmological Association of Australia's grading system.
Pattern regularity and coverage: A brick pattern that covers the entire face of the stone uniformly, with consistent patch size and clear coursing, is preferred over one that is confined to a portion of the stone or interrupted by patches of common opal (potch).
Patch size: Larger, more distinct patches are generally more desirable than small ones, as they are more visually legible and approach the harlequin ideal.
Directionality: A pattern that displays well from multiple viewing angles, rather than only from a single precise angle, is more commercially practical and therefore more valued.

Cutting Considerations

Orienting a brick pattern opal for cutting requires careful examination under fibre-optic or diffuse lighting to identify the plane of the colour bar and the axis along which the pattern is most regular. The cutter aims to orient the finished stone so that the table is parallel to the colour plane, maximising the face-on display of the pattern. Because the colour bar in many Lightning Ridge black opals is thin, cutters must balance depth of colour against the risk of cutting through the bar entirely. Domed cabochons are standard; flat-topped stones are occasionally cut when the colour bar is sufficiently thick and the pattern particularly fine, as a flatter table can enhance the geometric legibility of the brick arrangement.

Doublets and triplets — composite stones in which a thin slice of precious opal is bonded to a dark backing (doublet) or additionally capped with a transparent dome of quartz or glass (triplet) — are sometimes constructed using brick pattern material that is too thin to yield a solid stone. These assembled stones must be clearly disclosed in trade.

Care and Durability

Opal has a hardness of 5.5–6.5 on the Mohs scale and a relatively low toughness due to its amorphous, hydrated silica composition (SiO₂·nH₂O, with water content typically ranging from 3 to 21 per cent by weight). Brick pattern opals, like all precious opal, are susceptible to crazing — the development of surface or internal cracks — if subjected to rapid temperature changes or prolonged dehydration. Stones should be stored away from strong heat sources and direct sunlight for extended periods. Ultrasonic and steam cleaning are contraindicated; gentle cleaning with a soft, damp cloth is recommended.