New treatment could damage conch pearls

Basel--A Swiss gemology laboratory is warning the trade about a new coating being applied to the surface of conch pearls that does visible, nearly irreversible damage to them.

According to a news release from the Swiss Gemmological Institute (SSEF), a client recently submitted a necklace of conch pearls set between alternating seed pearls and diamond briolettes. The client said the piece had been worn only occasionally over the past few months, spending most of that time in his safe, and had been handled with the appropriate care.

Still, some of the pearls showed visible signs of damage. They were corroded with a whitish crust covering their surface.

A scanning electron microscopic analysis of the conch pearls revealed that they had been coated by a carbon-rich layer, i.e., an artificial resin. SSEF Director Michael Krzemnicki said that it is the lab’s presumption that the de-gassing of the coating caused the corrosion of the underlying conch surface.

“This process, in the end, results in the precipitation of a whitish encrusting on the surface. It seems that only a thorough reshaping and re-polishing can remove the damage,” he said.

When SSEF first examined the necklace, it appeared that the surface alteration had not affected all of the conch pearls. Further study, however, revealed that the coating alters the conch pearls in stages. They begin looking dull, then move to having distinctly altered (whitened) surfaces and, finally, become heavily encrusted with whitish precipitates.

According to the lab, Raman spectrometry is the preferred method for detecting this treatment. Even coated conch pearls that are visually still “perfect” display a very strong and characteristic broad luminescence band that is not present in uncoated conch pearls.

Krzemnicki said the lab has added this testing to its standard analytical procedures for conch pearls. On its reports SSEF will describe these stones as, “treated conch pearl,” with this additional statement: “Indications of surface coating. This coating is not stable and may deteriorate the conch surface in the course of time.”

He said one the dangers of this new treatment is that it might begin to show its effects only after a piece of jewelry is sold to a private client, which could, in turn, impact the consumer’s confidence in pearls and the jewelry trade in general.

“By clearly labeling this new and unstable treatment on our reports, we are determined to support the trade to