How do gemological laboratories test diamonds for certification?

While each gemological laboratory has its own standards and procedures, this answer focuses on the Gemological Institute of America (GIA). The GIA is one of the best-known gemological laboratories, and GIA makes no secret of its diamond testing and certification procedures.


When the GIA receives a loose diamond, it separates the diamond from anything that might reference the diamond’s owner—logos, letterheads, packing materials, etc. All diamonds are placed in individual transparent storage cases, each with a GIA identifier. In this way, the GIA assures the diamond graders make no subliminal connection between an individual diamond and its owner.

Each diamond is electronically tracked as it makes its way through the GIA diamond certification process. Diamonds that do not already have an inscribed GIA report number can have one microscopically inscribed on the diamond girdle at the client’s request.

After each grader’s inspection, the individual diamond is returned to GIA’s Inventory Control Department. This group coordinates the distribution of diamonds for evaluation, making certain the diamond progressively visits four different diamond graders, each grader more senior than the previous. The GIA Inventory Control Department also routes the diamonds so that exactly who evaluates which diamond is somewhat random, selecting from the hundreds of diamond graders working in each GIA laboratory.

After all four diamond graders have entered their assessments into the GIA software, a report is generated and matched with the corresponding diamond. At that point, client anonymity is no longer an issue. The evaluated diamond and its report are prepared for return to the client.

Carat Weight

The diamond’s first stop is the GIA Weights and Measures Department. A highly accurate electronic micro-balance weighs diamonds to the fifth decimal place. All of a diamond’s measurements, proportions, and facet angles are captured with an optical measuring device. All of the information is entered into the GIA’s operations and information management system, Horizon.

The weight and measurements are added to the label used for the diamond’s transparent storage case. Each case is labeled with a GIA internal identification number, as well as the specific services requested for that particular diamond.

The diamond is next sent for testing to determine whether the diamond is real or a synthetic substitute. The tests also look for the possibility of diamond enhancements, which mask a diamond’s original condition. The GIA uses Fourier transform infrared spectrometry (FTIR) and Raman spectroscopy to determine a stone’s identity and color origin. This equipment also evaluates colored diamonds.


The first diamond grader identifies finish and clarity characteristics under 10x magnification. This examination includes looking for filled fractures and laser drilling, as well as tests designed to alert the grader to whether a stone has been cosmetically treated or is possibly not a real diamond.

The GIA has a database of hundreds of diamond diagrams. The first diamond grader selects the diagram closest to the diamond’s actual shape and plots all clarity features on the diagram. The grader verifies information regarding the diamond’s measurements and weight and then assigns the diamond specific values for clarity, polish, and symmetry. Next the grader provides written descriptions of the diamond’s culet and girdle thickness. All of the grader’s observations are entered into a GIA software system where diamonds are only known by their GIA identifiers; they are not associated with any particular client.

A second diamond grader independently repeats all of the first diamond grader’s steps. At the end of inspection, all information is compared, though not by the same two graders to eliminate any bias.

In some cases, senior staff members are requested to make additional independent evaluations. These requests are triggered by a diamond’s weight, quality, and differences in the first two grading opinions.


All diamonds are evaluated for color in a standardized viewing environment. At least two color graders evaluate the diamond without knowledge of each other’s opinion. Additional color graders are asked for their independent color opinions, depending on the agreement of the first two assessors as well as the diamond’s carat weight and overall quality.


A GIA cut grade is assigned, taking into account the diamond’s clarity and color evaluations, measurements of facets and their angles, and descriptions of polish and symmetry. Other considerations include the diamond’s weight, durability, and scintillation, the sparkle and pattern as light hits the diamond. The GIA only provides a cut grade for standard round brilliant diamonds with color grades from D to Z.