What makes this a vg cut instead of ex?

[Minimesuv]

GIA cert 1156960599
5.81x5.84x3.42. Cut VG
Dep 58.7 tab 61 cr a 34 pav a 40.8
Pol vg. Sym vg. Cul none. Grd m-stk fir n
It is very sparkly to me hca 1.9
Should I still move on for ex cut?
THx so much

 

[Just_Starting]

Girdle width is within excellent parameters, the HCA score fits within the plotted GIA excellent zone. Since the polish and symmetry are both very good as well this means it could have been an excellent cut. I'm a bit confused lol this is normally pretty obvious. I wonder if the HCA is a little off and checking the dimensions against the GIA charts might explain it. I'm sure someone more experienced than I will be able to help!

 

[Minimesuv]

Thx for input, I am a newbie as well. I really think the stone presents itself ex . It is a strong choice for me but wanted ps'ers thoughts.

 

[Just_Starting]

No problem sorry I can't be of more help. I looked at these GIA charts and it fits within the parameters (just) of an excellent cut here too:
http://www.diamondcut.gia.edu/pdfs/booklet_cut_estimation_tables_lowres.pdf

Some good information here:
http://www.diamondcut.gia.edu/pdfs/estimating-a-cut-grade-hires.pdf

Alas, I didn't find anything that explains it. The diamond does seem to be borderline for many of the requirements so perhaps it was just too borderline and didn't quite make the mark, I'd be surprised if this was the case I imagine there is probably a more definitive explanation.

This part of the second link might explain it:

The actual overall cut grade received from the GIA Laboratory for a diamond
may differ from that predicted by the GIA Facetware®
for the following reasons:
• The diamond’s actual measured proportions (e.g., the crown angle) or grading
parameters (e.g., the symmetry) as determined by GIA are different than those used
as input by Facetware®
.
• The accuracy and tolerances of different measurement methodologies (including
optical) that are used in the trade may differ from those used by GIA to measure
proportions.
• The displayed girdle thickness and/or total depth (when not entered by the user)
is derived from other input proportions. In some cases, this may lead to girdle
thickness and/or total depth values that are different than the actual values due
to rounding and/or conversion from verbal descriptions (culet size and girdle
thickness).

 

[Minimesuv]

Thx for the links

 

[Roqsteady]

For one, it's a 60/60 diamond, and a pretty shallow one at that. It could be the shallow depth. The other thing is I don't know if you can have an ex cut but vg for both polish and symm on the gia scale. I know you can have one, not sure about two. If it's possible I haven't seen many if at all.

 

[Just_Starting]

VG symmetry and polish is possible to qualify for GIA excellent cut still.

 

[Minimesuv]

It is a VG cut, VG pol, VG sym. I thought it might b a 60/60 which is brighter and white vs fire

 

[gemmyblond]

It may well have to do with the 85% length of the lower girdle facets ("lower halves") on this stone. That's pretty long, especially when you factor in that GIA rounds to the nearest 5%. The cut charts assume an 80% lower girdle facet length, I believe, to predict cut grade.
So the long lower halves with this combination of angles may knock the cut grade down to VG.

But I'm not an expert, so you'd need to confirm this. Just my semi-educated guess(!).

 

[Minimesuv]

Thank you for ur thoughts.

 

[John P]

The basic lookup charts would cite this diamond as EX. But the combination has too many that are at-or-over the edge.

It may well have to do with the 85% length of the lower girdle facets ("lower halves") on this stone. That's pretty long, especially when you factor in that GIA rounds to the nearest 5%.

Both of these comments are key.

For the first comment: Table + lower-halves (+ stars to a lesser degree) have an important relationship, much like crown & pavilion angles. With 80% LH it would be EX. With a 59 Table it would be EX. Even with stars shortened to 50% it would be EX.

For the second comment: The rounding makes it impossible to know what the actuals are. I'm going to guess a number of factors were outside the given data on the wrong-side. In fact, with that depth, I suspect actual CA and/or PA average to be on the shallow side of the rounded numbers.