The flagstone pattern is one of the most visually striking and least common expressions of play-of-colour in precious opal. It is characterised by broad, irregular polygonal patches of spectral colour — red, orange, green, and blue — arranged across the face of the stone in a manner that closely resembles the irregular geometry of paving stones or flagstones set into a courtyard floor. Each colour block behaves as a discrete optical unit, flashing independently as the stone is rotated under direct light, producing a bold, graphic effect quite unlike the fine, dot-like scintillation of pinfire opal or the sweeping gradients of broad-flash.

Optical Mechanism

Play-of-colour in opal arises from the diffraction and interference of visible light by a three-dimensional lattice of amorphous silica spheres (opal-A) arranged in a regular, close-packed structure. When sphere diameter and spacing fall within the range of approximately 150 to 400 nanometres, the lattice diffracts white light into its spectral components at angles governed by Bragg's law. The specific pattern of play-of-colour — whether pinfire, broad-flash, rolling flash, harlequin, or flagstone — is determined primarily by the size of coherent diffracting domains within the stone: contiguous regions in which the sphere lattice is sufficiently uniform to act as a single diffracting unit.

In flagstone opal, these coherent domains are large — typically several millimetres across — and bounded by abrupt discontinuities in sphere orientation or packing. The result is that each domain diffracts a single dominant wavelength across its entire area, producing a solid block of colour with relatively sharp edges. Because adjacent domains differ in sphere orientation, they diffract different wavelengths simultaneously, creating the characteristic mosaic of contrasting colours. The angularity of the boundaries, rather than the smooth gradients seen in broad-flash material, gives flagstone its distinctive paved appearance.

Distinction from Related Patterns

Gemmological literature recognises a loose hierarchy of play-of-colour patterns, broadly ordered by the size of the colour patches they display. Pinfire (also called pinpoint) sits at one extreme, with colour patches so small they resemble a scattering of coloured sparks. Broad-flash occupies the middle ground, with large sweeping sheets of colour that roll across the stone as it moves. Flagstone is distinguished from broad-flash principally by the angularity and discreteness of its colour areas: where broad-flash colour tends to merge and flow, flagstone colour is compartmentalised into clearly demarcated blocks with visible boundaries between them. The harlequin pattern — widely regarded as the rarest and most valuable of all opal patterns — is sometimes considered a geometric sub-type of flagstone in which the blocks approach a regular, near-rectangular or diamond-shaped tessellation; in practice, the boundary between an exceptional flagstone and a true harlequin is contested among dealers and gemmologists.

Principal Sources

Flagstone pattern opal is most closely associated with Lightning Ridge, New South Wales, Australia, the world's pre-eminent source of black opal. The dark potch (common opal) body colour of Lightning Ridge material provides the ideal dark background against which large colour blocks achieve maximum contrast and brilliance. The specific geological conditions at Lightning Ridge — the slow, low-temperature silicification of voids within Cretaceous sedimentary sequences — appear to favour the formation of large, well-ordered silica sphere domains, which predisposes the deposit to producing flagstone and harlequin patterns at a higher rate than most other opal localities.

Flagstone pattern has also been documented in opal from Coober Pedy and Andamooka in South Australia, though the lighter body colours of these white and crystal opals reduce the visual drama of the pattern compared with black opal equivalents. Ethiopian opal from the Welo (Wollo) province, which has risen to commercial prominence since approximately 2008, occasionally displays flagstone-like patterning, though the hydrophane character of much Welo material introduces additional variables in optical behaviour.

Valuation Considerations

Within the opal trade, play-of-colour pattern is one of several primary valuation criteria, alongside body tone, brightness, colour range, and the dominance of red in the spectral display. Flagstone pattern commands a significant premium over pinfire of equivalent body tone and brightness, reflecting both its relative scarcity and its aesthetic impact. The most valuable flagstone specimens combine a black body tone (rated N1 to N4 on the standard body-tone scale), a brightness grade of B4 or B5 (brilliant), a dominant red component in the colour blocks, and well-defined, sharply bounded patches covering the majority of the stone's face.

Stones in which the flagstone blocks are large relative to the overall stone size are generally preferred, as this maximises the graphic clarity of the pattern. Conversely, very small stones may display what appears to be a flagstone pattern simply because only one or two colour domains are visible, which some dealers consider insufficient to constitute a true flagstone display. There is no universally adopted minimum patch-size criterion in published grading standards, and assessment remains substantially subjective.

In the Trade

The term "flagstone" is in common use among Australian opal miners, dealers, and specialist collectors, and appears in the catalogues of major auction houses when describing significant Lightning Ridge black opals. It is also employed by gemmological laboratories — including the Gemmological Institute of America and the Swiss Gemmological Institute (SSEF) — in their opal identification and quality reports, where pattern description forms part of the standard commentary. Buyers should be aware that pattern terminology is not yet fully standardised across all issuing laboratories, and that descriptions such as "large broad-flash" or "mosaic" may be used by some laboratories for stones that others would classify as flagstone.

Because flagstone and harlequin patterns are the most commercially desirable opal patterns, the terms are occasionally applied loosely in retail contexts to stones that more accurately display broad-flash or irregular colour distribution. Prospective purchasers of significant flagstone opals are advised to seek laboratory documentation and, where possible, to examine the stone under consistent, direct light — a fibre-optic or LED spot source held at varying angles — to confirm that the colour blocks retain their discrete, bounded character across the full range of viewing positions.

Further Reading

• GIA Gems & Gemology — Opal
• GIA: Factors Affecting Opal Value
• International Gem Society — Opal