ISO 24016:2020, the first international diamond grading standard

What ISO 24016 specifies

ISO 24016:2020 specifies the four grading criteria for natural polished diamonds, the 4Cs, and the methods to be used for each (ISO 24016:2020, iso.org/standard/79795.html):

Carat weight: Mass of the polished diamond, measured in metric carats (1 ct = 0.200 g) to the nearest 0.01 ct.

Colour: Assessed face-down against master stones, graded on the D-to-Z scale (D through L for the main scale; M through Z for progressively more coloured stones). The standard specifies that master stones be of "first or second generation" calibration, meaning they reference back to a primary standard rather than independently assessed commercial stones.

Clarity: Assessed under 10× magnification, with the same FL through I classification structure as GIA and CIBJO Blue Book. ISO 24016 notes that laser inscriptions on the girdle may affect the clarity grade assessment under specific circumstances, a specific provision not commonly discussed in commercial grading contexts.

Cut: Assessed for proportions, symmetry, and polish using the same Excellent/Very Good/Good/Fair/Poor scale used by GIA and referenced in CIBJO. The standard provides specific proportion ranges for each grade category.

Scope: what ISO 24016 does and does not cover

ISO 24016:2020 explicitly applies only to (ISO 24016:2020, iso.org/standard/79795.html):

Natural, unmounted, polished diamonds above 0.25 carats.

It explicitly does NOT apply to: fancy coloured diamonds (which require a different grading methodology based on hue, saturation, and tone); synthetic (laboratory-grown) diamonds; diamonds treated by methods other than laser drilling; assembled stones (doublets, triplets); mounted diamonds (in settings); and diamonds of 0.25 carats or below (which will be covered by the forthcoming ISO 6893).

The removal of the one-grade tolerance

One of the most commercially significant aspects of ISO 24016 is what it does not include: the traditional one-grade tolerance. The diamond trade had informally accepted for decades that a grading laboratory might be "one grade off" from another laboratory on borderline stones, that is, two reputable labs could legitimately assign adjacent grades (G and H, for example) to the same stone. This tolerance was understood to reflect genuine uncertainty at grade boundaries.

ISO 24016 does not incorporate this tolerance. The standard specifies that grades should be assigned based on the defined criteria without reference to allowable variance from another lab's assessment. JCK Magazine noted in its analysis of ISO 24016 that "it does not allow for the traditional one-grade tolerance, a bit of received trade wisdom that allowed for labs to be one grade off from each other" (JCK, "Will The ISO Universal Diamond Grading Standard Matter?", October 2020).

The practical implication: if a laboratory claims ISO 24016 compliance, it is committing to achieving accurate grades, not grades that are merely within one step of a reference. This raises the bar for claimed compliance, which is one reason ISO 24016 compliance declarations are not yet widespread.

Laboratory compliance: voluntary and currently limited

ISO 24016 compliance is voluntary. A laboratory that voluntarily adopts the standard can state "graded according to ISO 24016" on its reports, but doing so also requires compliance with ISO 17025, the general laboratory accreditation standard. A laboratory may further choose to have a third-party accreditor review its diamond grading procedures to confirm ISO 24016 compliance, and can note this on its reports (JCK, op. cit., 2020).

As of 2024–2026, GIA and IGI do not routinely state ISO 24016 compliance on their standard reports. Both laboratories' grading methodologies are broadly consistent with ISO 24016 criteria, the standard was built on the same framework they use, but the formal compliance declaration with its associated ISO 17025 requirement has not been universally adopted. SSEF (Swiss Gemmological Institute) committed to applying the standard at its launch (JCK, op. cit., 2020). Verify current compliance declarations with specific laboratories before citing them.

ISO 6893: the next step, melee diamonds

A third ISO diamond standard is in development: ISO 6893, which will address quality control of diamonds under 0.25 carats in batch form. This standard is being developed for the Swiss watch industry's need to assess large parcels of melee diamonds using statistical sampling (Acceptable Quality Level methodology) rather than individual stone grading (CIBJO, "New standards being developed to control quality of batches of small diamonds," cibjo.org/industry-standards-session, 2023). ISO 6893 was under development as of 2024, verify current status at iso.org.

Primary sources