In most of the diamonds we have analyzed to date that exhibit "digging", we have generally found it to be among the upper girdle (also referred to as upper half) facets which are angled further away from it's neighboring main facet and more towards each other causing a steeper angle in the half facet as a result. To quote GIA; "...the half facets can be fashioned so that they lean toward each other, thereby creating steeper angles between each upper or lower girdle facet and its neighboring bezel or pavilion main facet, and steeper angles of the halves themselves with respect to the horizontal (in which case they are referred to as “dug out” facets). These techniques are well known in the diamond cutting industry, but seldom discussed outside of that group."
If that was a little tough to understand, the graphics below will help you to better grasp the concept and what it is that is being looked at. The diamond used in this example is one that just falls outside of the AGS Ideal range and will be submitted in January to see how it qualifies on the new GIA Cut System (we don't anticipate Excellent grade). Below is a look at the first piece of data and the first graphic is of a stone with dug out upper girdle facets next to the classic ideal girdle. Note how the girdle thickness from bezel to half is not consistent as it gets thinner at the half.
Here's a zoomed in view of this comparison. It's easy to see how the girdle thickness at the halves (bone) are thinner and not consistent like the classic girdle.
The girdle graph as generated from a Helium scan on this diamond shows us this. The graph shows the obvious difference with the girdle half facets at the crown being pushed down not maintaining even thickness around the diameter of the stone.
When we compare girdle measurements via Sarin we can note this difference numerically. Note now the difference between the girdle thickness at the bezels as compared to the girdle thickness at the halves.
Here is side by side next to the ideal classic girdle. Note the difference in girdle thickness between the halves and bezels among these 2 stones. Also take note of the upper girdle and lower girdle angle relationship. On the Classic Girdle we have a 42/42 relationship while the diamond with the dug out girdle has steeper upper girdle angles (45.3°). This is what GIA was referring to when they state "the half facets can be fashioned so that they lean toward each other, thereby creating steeper angles between each upper or lower girdle facet and its neighboring bezel or pavilion main facet, and steeper angles of the halves themselves with respect to the horizontal (in which case they are referred to as “dug out” facets)."