Overtone (Pearl Colour Factor)

The translucent secondary colour that floats over the pearl's body

Overtone, in the GIA pearl description system, is the translucent secondary colour visible on the surface of a pearl, lying optically over the body colour and arising from the interference and diffraction of light at the boundaries between successive nacre layers. Common overtones include rosé, silver, green, blue, and gold, and the strength and quality of the overtone is one of the principal contributors to the visual character and the market price of a fine pearl.

The optics of overtone

The nacre of a pearl is a layered composite of aragonite platelets and conchiolin, with the platelets typically a fraction of a micrometre thick and stacked in regular succession. Light entering the pearl is partly reflected at each interface and partly transmitted to the next layer. The reflected light from successive layers interferes to produce the diffraction-derived colour that an observer sees as the overtone, with the precise hue determined by the thickness and regularity of the aragonite platelets and by the wavelength of the incident light.

The body colour, by contrast, is the dominant colour seen through the pearl's nacre and is determined by the pigments deposited in the nacre and by the colour of the underlying pearl-sac tissue and bead nucleus where present. Body colour and overtone are independent: a white-bodied pearl may carry a strong rosé, silver, or green overtone, and a black-bodied Tahitian may carry a peacock, aubergine, or pistachio overtone.

The standard overtones

The overtones recognised in the GIA system include rosé (pink), silver, green, blue, and gold. A pearl may carry a single overtone or a combination, with the strongest stones showing a clean dominant overtone and a secondary overtone visible at oblique viewing angles. White Akoya pearls characteristically carry a rosé or silver overtone, with the rosé-overtone material historically the highest-priced commercial Akoya. White South Sea pearls range from silver through to a soft pinkish-silver. Tahitian pearls carry the broadest overtone range — rosé, green, blue, peacock (a green-rosé combination), aubergine, and pistachio — and the precise overtone is one of the principal pricing factors for Tahitian material.

Overtone strength and quality

Overtone strength is graded subjectively as none, faint, moderate, or strong. Strong overtones are visible from a wide range of viewing angles, while faint overtones are visible only under direct light and at limited angles. The quality of the overtone — the cleanliness and saturation of the colour rather than the strength of its appearance — is also assessed, with cleaner, more saturated overtones grading above muddier or less defined ones.

Overtone tracks closely with lustre and nacre quality. A pearl with thick, regular nacre will produce a strong, clean overtone, while a pearl with thin or irregular nacre will produce weak or absent overtone. The two factors — overtone and lustre — are therefore correlated, and the highest-priced pearls are those that score well on both.

In the trade

Overtone is one of the seven factors in the GIA pearl description system, alongside size, shape, colour (body colour), lustre, surface, nacre, and matching. CIBJO and the principal Asian laboratories follow comparable conventions. For Akoya, white-with-rosé commands the strongest premium; for Tahitian, peacock and rosé overtones command the strongest premium; for South Sea, the value gradient is more weighted toward silver, gold, and white-with-silver-overtone material.

Overtone strongly affects matching cost in strands and pairs. A matched strand of pearls with consistent overtone is significantly more expensive than a strand of comparable size and lustre with overtone variation, because the matching process discards a high percentage of otherwise good pearls.

Treatment and disclosure

Overtone in fine pearls is the natural result of nacre layering and is not enhanced by treatment. Some lower-grade material is dyed or irradiated to alter body colour, and dyed pearls typically show artificial-looking colour without the natural diffraction-derived overtone of untreated material. Treatment must be disclosed under CIBJO and AGTA terminology.