The Gemological Institute of America’s certification laboratory suspends its diamond sealing service for the second time in six years. A suspected fraudster has allegedly replaced untreated diamonds in a sealed package with treated stones.

The suspension, effective immediately, was in response to the discovery by the laboratory that an unspecified “small quantity” of its sealed packages had been compromised after leaving the GIA. According to the GIA, the unprocessed diamonds originally contained in the parcels had been replaced by natural diamonds processed under high pressure and high temperature (HPHT). The treated stones were physically and visually similar to the original stones, a spokesman said.

A sealed diamond is placed in a package, which must be secure and tamper-proof. The laboratory seals the diamonds as an additional service, usually for the investment market. Barely one-tenth of 1% of stones certified by the GIA are sealed.

The laboratory always investigates the “source and method” of the alleged fraud, including how the package was broken.

This is the second time the GIA has suspended its sealing service. The first time, in 2014, the laboratory had discovered that a sealed diamond had been replaced by a lower quality stone. The service resumed several months later, after additional security measures were put in place.

Anyone with concerns about a sealed diamond can present the stone to a GIA laboratory for a free verification.

If the GIA concludes that the diamond in the package is the right diamond, it will write a letter confirming that the diamond matches the original report. If the stone does not match the certification report, the GIA will issue a new report with the correct information. All sealed diamonds presented to this verification service will be returned unsealed, as the GIA has suspended the service.