In summary, for reliable diamond grading, the top recommendations are AGS and GIA, which are widely regarded as trustworthy and consistent. HRD and GCAL are also considered reliable labs. The International Gemological Institute (IGI) has been making improvements in its diamond grading process. However, the European Gemological Laboratory (EGL) has faced a loss of credibility and market share due to inconsistencies. The Gemological Science International (GSI) is generally satisfactory, though occasional grade inconsistencies may arise. The Professional Gem Science Laboratory (PGS) is dependable but less known in the market.

AGSL - AGS Certification
The American Gem Society (AGS) functions as a membership organization encompassing jewelers, suppliers, appraisers, and traders. Its inception dates back to 1934. However, its laboratories, known as the American Gem Society Laboratories (AGSL), were established in 1996. The organization also features an advanced instruments division, founded in 2004. AGSL is primarily responsible for grading diamonds and has developed its own set of standards. It stands as a formidable rival to the GIA and holds a strong reputation within the diamond industry.
Yet, an area of concern arises when AGS grades the clarity of diamonds, particularly in the SI1/SI2 range. Instances have been observed where AGS graded diamonds in the SI2 category as if they were more akin to I1 diamonds. Consequently, caution is advised when dealing with AGS's inconsistencies in grading clarity within the SI2 range.
Please note AGS merged with GIA in late 2022. As such, these insights are relevant to existing AGS grading reports.
GIA Certification
The Gemological Institute of America (GIA) operates as a non-profit organization focused on the research and education of consumers regarding jewelry and gemstones. Their involvement encompasses the study and grading of diverse gemstones. Alongside their lab services, GIA offers training for gemologists and provides comprehensive education and research support to the diamond industry. GIA's diamond grading reports are highly esteemed as the foremost "certifications" and verification documents in the industry.
GIA offers two distinct types of diamond grading reports: full reports and diamond dossiers designed for smaller diamonds. Dossiers are more cost-effective compared to full reports, yet they lack the complete plot of a diamond, omitting the precise location of inclusions. In the case of dossiers, GIA mandates a laser inscription on the diamond, given the absence of inclusion locations. Conversely, full reports offer a different approach. Diamond dealers have the option to include a laser inscription as an additional service within full reports, for an added fee of $30.
However, with GIA's expansion, concerns have arisen regarding the declining quality of their grading services. Instances have emerged where GIA categorized a G-color-graded diamond as a lower H color.
Discussions also revolve around GIA's methodology for determining excellent cuts in diamonds. A point of contention pertains to the broad criteria employed when assigning excellent cut grades to diamonds – the highest tier on GIA's scale. In comparison, it has been suggested that AGS's utilization of a proportion-based system for assigning ideal cut grades in their reports may yield a superior grading approach. For those considering well-cut GIA graded diamonds, it's advisable to delve into diamond proportions and refer to a chart.
HRD Certification
HRD Antwerp, closely linked to the Antwerp World Diamond Center, occupies a significant role within the diamond trading and supply sector. Established in 1973, HRD represents the Belgian diamond industry. The HRD Lab adheres to standards akin to those of GIA and follows grading regulations set by the International Diamond Council, underlining its reliability.
Nevertheless, it is worth noting the possibility of grade inconsistencies in HRD's grading process, particularly in relation to diamond color and clarity. This aspect warrants consideration when contemplating the purchase of HRD-graded diamonds.
IGI Certification
Founded in 1975 and headquartered in Antwerp, the International Gemological Institute (IGI) has a global presence with multiple branches and a gemology school. However, IGI's widespread reach translates to slightly varying grading criteria among their labs in different countries. While still a major player, IGI's accuracy is generally considered somewhat lower compared to GIA or AGS. Anticipating one-grade inconsistencies when purchasing IGI-graded diamonds is common. Recognizing the uniqueness of each diamond, a generalized approach is less effective as individual cases require tailored consideration.
As such, a degree of grading inconsistency across all 4Cs is to be expected when it comes to IGI's grading.
GSI Certification
Gemological Science International (GSI) primarily serves to grade diamonds for large chain stores, with a focus on high-volume grading. With a global expansion, GSI's appeal to large stores lies in their ability to expedite diamond grading compared to GIA or AGS. While their accuracy is comparable to IGI in diamond grading, GSI often assigns slightly lower grades compared to GIA or AGS. Nonetheless, due to the distinct characteristics of each diamond, individual evaluation remains crucial.
For those without comprehensive knowledge or the time for exhaustive research, opting for either GIA or AGS certifications is advisable.
Prudence is recommended when assessing GSI-graded diamonds, as some degree of grade inconsistency may occur. Seeking the expertise of a third-party expert can be beneficial, considering that the reliability of appraisers can vary. This may entail trade-offs when acquiring GSI-graded diamonds.
EGL Certification
The European Gemological Laboratory (EGL) once held a significant stake in the diamond grading realm. However, its credibility has suffered, resulting in a decline in reputation. EGL is currently undergoing reorganization, leading to skepticism about relying solely on their reports. Due to its diminished reputation, platforms like RapNet have chosen to exclude EGL-graded diamonds from their listings.
Given these circumstances, exercising caution when considering EGL-graded diamonds is prudent, particularly unless they are from the 1950s to the early 2000s. Even within this time frame, up to two grade inconsistencies in overall grading may be anticipated.