The trade is being warned to be on the lookout for a new pearl coating treatment that damages the gems’ surface—possibly permanently.
The Swiss Gemmological Institute (SSEF) detected the previously unknown treatment after running tests on a necklace with seriously corroded conch pearls. It detected an artificial resin coating that improves the pearls’ appearance but seriously damages their surface in the long term, potentially irreversibly.
The necklace in question had been purchased in the last few months, then sat mostly in the owner’s safe, SSEF said.
"Obviously, the corrosion process and the ensuing damage caused to some of the pearls in the necklace occurred while the pearls were in the safe," said SSEF director Dr. Michael S. Krzemnicki in a statement. "When the owner removed them, some of the formerly perfect-looking conch pearls showed a completely corroded surface, which profoundly affects their lustre and color and their beauty.”
The pattern of the corrosion rules out “normal” damage (e.g., resulting from acids or perfume).
Scanning electron microscopic analyses of the conch pearls revealed they were coated by a carbon-rich layer. The lab now presumes the corrosion was caused by the de-gassing of the coating.
The treatment is very difficult to detect even with a microscope, the lab said—although clues include small glue-like bulges at the gems’ surface and brownish granular spots below the surface. It said the preferred identification method is Raman spectrometry, as the pearls exhibit a strong and characteristic broad luminescence band compared with untreated conch pearls.
If SSEF finds more pearls with this treatment, it will label them as “treated conch pearl,” with an additional comment: “Indications of surface coating. This coating is not stable and may deteriorate the conch surface in the course of time.”