GIA also makes a habit of putting micro-laser inscriptions on the gems it grades. You would not see these engravings with the naked eye, because they are microscopically small.
Even under 10x magnification, it is hard to spot if you do not know what you are looking for. The engraving is always made on the girdle of the diamond, which is its outer edge. If you have this number, it can be entered into the GIA website to get an electronic copy of the grading report.
Finding the Girdle
A diamond’s girdle runs around the outer edge in the area that falls between the upper and lower facets. It is an outer edge, which separates the upper section from the lower part of the diamond. It can be bruted, faceted, or polished in appearance.
The laser inscription from GIA assures a consumer of the quality of the GIA certified diamond ring when they purchase it. Some laboratories other than GIA make similar markings, but not all. Once an inscription is made, the number is registered permanently in the GIA database.