Bleaching
Pearls are routinely bleached with hydrogen peroxide to lighten them and even out their color. One-step bleaching is virtually impossible to detect in most cases (GIA). GIA suggests keeping pearls in a soft, dry environment.
Dyeing
Dye improves the color of lower-quality pearls, and many pearls on the market are dyed. Dye can be removed by alcohol or acetone and can fade with prolonged sun exposure (GIA).
How to spot it: dye often concentrates in the drill hole and in pits or cracks. Uniformly pink akoyas may show pink in the hole. Dyed freshwater pearls often have unnatural colors you can see without magnification. A black or dark gray cultured pearl under 8 mm is likely a dyed akoya or freshwater pearl (Jewelry Business).
Irradiation
Some pearls are irradiated, producing dark gray colors (GIA). Irradiation can give a bluish or greenish gray tone, and it is sometimes combined with dye or silver nitrate (Jewelry Business).
Coating
Some pearls have reportedly been given a colorless hard coating to improve durability (GIA). Coatings can also be used to improve luster or change color; under magnification a coated pearl can look glittery and grainy, and worn coatings leave bald spots (Jewelry Business).
Bleached "chocolate" pearls
GIA studied the brown "chocolate" pearls of the 2000s and concluded they were black Tahitian cultured pearls whose organic components had been bleached. A strong brown with no rosé or orient overtone should prompt questions. See chocolate pearls.
Care for treated pearls
Keep dyed pearls away from alcohol, acetone and long hours in the sun. Coated pearls should be kept away from hard or abrasive objects.
