Articles about Diamond Grading

Gemstone cutters can tilt or 'index' the upper half ( or upper girdle ) facets. This indexing produces different amounts of weight retention from a given piece of rough and produces different optical performance effects. Indexing can also affect crown height and spread ( millimeter footprint versus weight, also called 'weight ratio' ).

In the course of this first quarter of 2005, AGS will launch new grading reports for princess-cuts. For the first time in the history of diamonds and grading labs, there will be a cut-grade on a fancy shape.

While we are preparing and cutting some princesses in order to get a 0-grade, we are amazed by the major changes that this new grading report demands. I will try to highlight these here.

As some industry leaders are fond of pointing out, a non-H&A diamond can be just as bright under direct lighting as some H&A diamonds.

I agree.

But that is not the whole story when it comes to H&A diamonds... After all, most people don't carry a set of halogen lights to shine on their diamond at all times. A diamond on the average persons hand is in indirect and or dim lighting much of the time. Therefore, the diamond that performs best under direct lighting isn't always the best diamond for the most common conditions.

A chess board is 50% black; it returns half as much light as a sheet of white paper. But a chess board looks brighter than white paper when they are both moving. The same is true for a diamond; if a stone returned light so that the crown was bright all over, it would not appear as bright to your eye s as one with good contrast. While light return is an important part of ‘brilliance’, our perception of brightness is enhanced by our eyes adaptation to contrasting backgrounds. All six patterns in Fig 1 are equally bright to a light meter, but those with good contrast are more brilliant to our eyes.

The diamond business is at a moment in time where old ideas and technology are truly meeting head on. We have the GIA and AGS about to grade the cut of a diamond based on measurements and ray tracing computations along with minor parametric grade limiting characteristics. We have several other players grading or about to grade with similar technologies. We have Gemex using six light positions in an additive way that does not mimic real world lighting and some other tools which compare data with appearance.