While many of these are not common in the market, GIA tests every diamond it grades for their presence.
Coating enhances a diamond's color by masking an undesirable body color with an ultra-thin layer of chemicals or plastics. Another form of coating involves applying a thin film of synthetic diamond to the surface of a diamond simulant, giving it certain characteristics of a real diamond.
HPHT stands for a high-pressure, high-temperature. The process is an effective tool for changing the color of certain diamonds, making them colorless, pink, blue, green, yellowish green, or yellow. Outside of a well-equipped grading laboratory, this form of treatment is virtually undetectable.
There are two main techniques for improving a diamond's clarity; laser drilling and fracture filling.
Laser drilling is commonly used to remove small dark inclusions. The laser bores a small hole into the diamond's interior and burns away the inclusion, or creates a channel through which a bleaching agent can be introduced to improve the inclusion's appearance.
Fracture filling hides white fractures in a diamond called "feathers." A glass-like substance is injected into the fracture to make it less visible and to improve the stone's apparent clarity. Because the filling may be damaged or removed during routine cleaning and repair, the technique is controversial. Good fracture filling is very subtle, and so examination by a skilled diamond grader is necessary to detect its presence in a stone.
How does a GIA Diamond Grading Report help you know whether your diamond has been treated?
GIA does not issue grading reports for any diamond that has undergone a treatment process that's considered nonpermanent or unstable, such as coating or fracture filling.
GIA will issue reports for diamonds that have been laser drilled or HPHT processed, prominently disclosing these treatments on the report. As a further precaution, GIA also laser-inscribes the girdles of diamonds it identifies to be HPHT processed.