Spectral Characterization of Mabe (Mabe-Pearls, Half-Pearls or Blister-Pearls) Using Integrating Sphere Spectroscopy

Scanning spectrophotometers equipped with integrating spheres were once highly specialized spectrophotometric equipment but are now readily available commercially. They are particularly useful for objectively assessing the spectral reflectance of objects, hence their value for assessing gemstones and semiprecious stones, including pearls. We measured the reflectance properties of mabe (sometimes called cultivated mabe-pearls, half-pearls, blister-pearls, or shell cultured blisters) produced by the pearl oyster Pteria penguin (Röding, 1798) grown in Phuket, Thailand, prepared commercially as mabe buttons (mean 13.6 mm diameter). Reflectances were similar in white and pink mabe but not gray mabe in the violet/blue 400–550 nm range (violet to green) precluding the use of those wavelengths for diagnostic purposes. Minimal reflectance was found at about 550–600 nm (yellow to orange). Diagnostic color features are found above 600 nm in the conventional red visible range (625–700 nm), far red 700–750 nm and in the near infrared (NIR) 750–850 nm. Pink mabe reflect strongly (R% >70%) in the visible red (625–700 nm), far red (700–750 nm), and NIR (750–850 nm) with a conspicuous point of inflection at about 625 nm (orange to red). White mabe reflects more strongly in the red part of the spectrum but less strongly than the pink mabe. Some white mabe showed a weak point of inflection at ≈625 nm. Gray mabe have a very different spectrum to pink and white mabe: a conspicuous minimum at ≈570 nm (yellow) and low reflectance similar to that found in the blue part of the spectrum at visible red, far red, and NIR wavelengths. Objective spectrophotometric classification of mabe pearls from P. penguin is a viable proposition.