Understanding the GIA and AGS Cut Grading Systems
In the cut grading systems, each of the major labs are considering aspects of diamond appearance, performance and measurement characteristics. In an effort to keep things as simple as possible we will look at the individual metrics considered from each lab and basically gel the teachings of each metric into one chapter.
The GIA Cut Grading System is a 5 grade system rating the diamond as either Excellent, Very Good, Good, Fair or Poor.
The metrics GIA is considering are those of:
- Face-Up Appearance
- Brightness: In the past used to be referred to as "brilliance”. Brightness specifically refers to the amount of bright white light return observed from the diamond as it is observed in diffuse lighting conditions.
- Fire: Refers to the display of colored light return as observed in spot lighting conditions.
- Scintillation: Simply refers to the movement of the light's pattern or sparkle within the diamond as the diamond, observer or light source is moved.
- Weight Ratio: Refers to the appearance of the size of the diamond in relation to its weight. Ie. Diamonds cut too deep appear to be smaller than their intended weight. There are different ways to retain weight in a diamond and GIA knocks a diamond with hidden weight. To quote from them, a “diamond with a thicker girdle or a larger total depth also can be attractive, but then the customer pays for unnecessary weight.”
- Durability: Refers to the possibility of the diamond chipping or breaking due to extremely thin girdles. GIA did not find evidence that a diamond’s durability suffered from shallow angles,unless the girdle was very thin.
- Polish: Quality of the finish ultimately affecting luster.
- Symmetry: Facet placement and their alignment with adjoining and opposite corresponding facets. You can read more about Polish and Symmetry on our tutorial here with a full listing of all the features that could possible go wrong in these metrics.
To summarize, here’s a quote from GIA: A diamond is attractive when it is bright and fiery; when it has a pleasing, even pattern to its areas of light and dark (scintillation); and when it demonstrates a high level of craftsmanship. A diamond is less attractive if it is dark, if it is not fiery, if it has a distracting or unpleasant pattern, if it is carelessly polished, or if it is sloppily cut. Also, a diamond with a too-thin girdle may be attractive, but it is more susceptible to damage. A diamond with a thicker girdle or a larger total depth also can be attractive, but then the customer pays for unnecessary weight.
AGS’ Cut Grading System is an 11 grade system that takes into account other parameters not considered in GIA’s system and is viewed as the stricter of the two. The highest grade in AGS’ system being “0” and the lowest grade being “10”. The metrics AGS is considering in their final cut grade can be broken down into 3 primary categories:
- Light Performance:
- Brightness: Same as above.
- Dispersion: Similar to fire in that it is the observance of colored light return through the crown in conditions that flatter this metric.
- Leakage: Facets which are functioning in a role of "window" rather than "mirror" or "non-reflector" as opposed to a "reflector". An important element in understanding how AGS arrives at a cut grade.
- Contrast: Before we begin to explore all the optical metrics that constitute diamond appearance, an understanding of contrast is vital as it plays an extremely important role in diamond appearance and more specifically personal preference. As we explore each of the metrics we will be considering the role of contrast and how it impacts each of these metrics and more specifically, how AGS identifies and defines it.
- Proportion Factors:
- Tilt: Not only is the face up view considered but tilting the stone to examine its optics are considered.
- Durability: Same as above.
- Weight Ratio: Same as above.
- Girdle thickness: too thin or too thick = no no
- Culet Size: must not be chipped or too large.
- Polish: Same as above
- Symmetry: Same as above.