Diamonds are graded by the Gemological Institute of America (GIA) or the American Gem Society Laboratories (AGSL). Grading precious gemstones and diamonds, these laboratories are two of the most respected and prestigious institutions in the world. Comparing diamonds graded by these two different laboratories is simple, as although they give different grading reports, they both apply similar criteria.

THE DIAMOND CUT GRADING CRITERIA BY GIA

The GIA grading criteria for a new diamond cut grade is given, based on a combination of criteria such as face-up appearance, design and craftsmanship. It is all these factors which then contribute to the brilliance and fire of a diamond. At GIA they use a predictive computer model which bases its evaluations on over 70,000 individual diamond observations, and 38.5 million proportion sets. Determining a gemstone’s brilliance based on its measurements, these criteria grade precious gemstones on a scale of excellent to poor. However, those graded by GIA prior to January 1^st, 2006 will not have been assigned a laboratory approved grade.

The GIA cut grading criteria:

• Face up appearance – white light and colored light reflections are all very important in the cut grading of a diamond. The criteria looks for:

o    Brightness – the reflections of white light seen when the diamond is viewed from above

o    Fire – the flashes of color refracted in the light; similar to those in a prism

o    Scintillation – patterns of light and dark areas created by light

• Design – the shape, size and durability of a diamond is also an important factor in the grading criteria.

o    Weight – relative to the diameter

o    Durability – resistance to chipping

• Craftsmanship – for a diamond to be awarded a high grade in this criterion, it must be finished perfectly. The ‘marks’ of polishing should invisible to the naked eye, and enhance the brilliance of the gemstone. Symmetry is also important, as lack of it may affect the diamond’s ability to reflect light.  

AGSL DIAMOND CUT GRADING CRITERIA

Similarly to GIA, the American Gem Society Laboratories grade the cut of a diamond according to its light performance, proportion and finish. It is each of these characteristics that contribute to the diamond’s brilliance.

When AGSL determines a grade of a gemstone or diamond, they take into account the following criteria.

• Light performance – the appearance of the diamond when viewed from above. This is broken down into these categories:

o    Brightness – the quality of the white light reflections

o    Dispersion – how well the white light gets broken down into spectral colors

o    Leakage – the areas where light escapes

o    Contrast – this is the patterns of light and dark created by the cut of the diamond, this can affect its brilliance both positively and negatively.

• Proportion factors – this is a summary of the decisions made when a diamond is crafted, these include:

o    Girdle – this is the perimeter of the diamond where the crown and pavilion intersect. N.B. You should avoid girdles which are very thin as this makes the diamond more prone to chipping. Similarly, precious gems with a girdle which is too thick may put too much weight in the middle of the gemstone, causing it to look smaller than diamonds of similar weight.

o    The culet – this is the facet or point at the bottom of the diamond’s pavilion. Cutlets in preferred grades are undetectable to the naked eye.  

o    Spread – this is the face-up appearance of the gemstone. Diamonds with Ideal cuts are equal in weight and size.

o    Durability – measures of a diamond’s resistances to chipping

o    Tilt – the point at which the gemstone’s girdle can be seen to reflect light onto a tabletop.

The AGSL grading criteria assign diamonds with cut grades similar to GIA including, Ideal, Excellent, Very Good, Good, Fair and Poor.