Diamonds certified by the GIA had an undisclosed treatment which fades over time, but improves their colour by as much as three grades.
GIA is asking anyone in trade stocking these potentially treated stones to turn them back into the GIA for re-examination.
The majority of the GIA certified diamonds are one carat or larger, with a number of three to five carat diamonds. A colour grade of three higher in these sizes amounts to a very big difference in price.
GIA has terminated the client agreements with companies suspected of knowing the diamonds were treated and did not disclose it.
GIA discovered the treatment when a client who purchased one of the diamonds resubmitted it to GIA for re-check.
It was then that the GIA connected this stone with hundreds of treated diamonds submitted by the companies.
The GIA has notified industry bodies about the still unidentified temporary treatment.