Peacock pattern is one of the descriptive pattern designations applied to opal play-of-colour, denoting large, saturated patches of blue, green, and violet diffraction colour arranged in a pattern reminiscent of peacock feathers. It is most strongly associated with black and dark opal from the Lightning Ridge field of New South Wales, Australia, though comparable patterns are produced occasionally at Mintabie and other Australian fields. Peacock pattern sits near the top of the opal pattern hierarchy alongside harlequin and broad-flash, and stones graded as peacock pattern command substantial premiums above pinfire, ribbon, and other less prized patterns.

Pattern in opal grading

Opal grading evaluates four principal factors: body tone (the darkness of the underlying matrix), brilliance (the intensity of the play-of-colour), pattern (the geometric arrangement of colour patches), and colour (the spectrum of diffracted colours present). Pattern is the second most subjective of these factors, after pattern's interaction with brilliance, and is graded by reference to a vocabulary that has developed within the Australian opal trade and been adopted internationally. The principal pattern names include pinfire, broad-flash, ribbon, rolling-flash, harlequin, and peacock.

Peacock pattern requires three things: large patch size — typically greater than three millimetres in the longest dimension; high saturation in each patch, with no washed-out or pale flashes; and a multicoloured palette dominated by the cool end of the spectrum (blue, green, violet) with sufficient warm-end colour (red, orange) for the pattern to rotate convincingly across the peacock palette as the stone is tilted. A pattern that meets the size and saturation criteria but lacks the cool-end dominance is graded as broad-flash rather than peacock; a pattern that meets the colour criteria but is composed of small patches is graded as pinfire.

The structural basis

Opal play-of-colour is produced by diffraction of incident light at regular arrays of silica spheres in the gem's microstructure. Sphere size determines wavelength of diffraction: larger spheres in the 200 to 350 nanometre range diffract red and warm wavelengths; smaller spheres in the 150 to 250 nanometre range diffract blue and cool wavelengths. Peacock pattern corresponds to opal in which sphere arrays at the smaller end dominate but not exclusively, producing a multicoloured palette skewed toward the cool end. The dark body tone characteristic of peacock-pattern stones comes from inclusions of common opal or other dark mineral phases in the surrounding matrix.

Lightning Ridge dominance

Lightning Ridge is the principal source of peacock-pattern opal. The field's geology — sedimentary host rock containing opal-precious nodules formed during Cretaceous-period diagenesis — produces the combination of dark body tone and large, saturated colour patches that define the peacock category. Mintabie produced significant peacock-pattern material during its operational period (the field has largely been mined out as of the late 2010s); Coober Pedy produces peacock-pattern stones occasionally, though its predominant output is light and crystal opal with patterns at smaller scales.

Pricing

Peacock-pattern black opal from Lightning Ridge in the top brilliance grades commands prices comparable to fine ruby and sapphire on a per-carat basis. Stones above one carat with N1 to N3 body tone, full peacock palette, and clean surface routinely exceed ten thousand dollars per carat at the wholesale level; auction comparables from Sotheby's, Christie's, and the specialist opal trade place top stones above twenty thousand dollars per carat. Commercial peacock-pattern stones at smaller sizes and lighter body tones are available at substantially lower price points.

Further reading

• GIA — Opal reference and pattern grading
• GIA Gems & Gemology — Lightning Ridge and Australian opal articles
• ICA — opal information
• International Gem Society — opal pattern references