HCA 0.5 vs. ideal

[sparkleslee]

Hi, long time lurker, first time poster here.

I have a diamond that scores 0.5 on the HCA. It was appraised by GIA before the GIA came out with their cut evaluation. I''m confused though, because on the HCA chart it actually is UNDER the ''AGS ideal candidate'' *and* the "GIA excellent candidate.''

It seems like most people have diamonds that are rated GIA excellent or AGS ideal but have high HCA, and their placement on the HCA chart is ABOVE the ideal/excellent candidates. What is going on???

1.39 carats
dimensions 7.24 x 7.28 x 4.36
depth % = 60.1%
table diameter 56%
crown angle 34.6 deg
pavilion depth 42.5% (no pavilion angle!)
culet = none
girdle medium, faceted
polish excellent
symmetry excellent

 

[milton333]

The lab definitions of what is "ideal" are based on mathematical formulas for very precise ranges of proportions in an rb. An "ideal" rating does not always mean the best light performance, and vice versa. For example, I have my eye on a GIA-cert''d rb that scores only a "very good" cut from GIA, but ex/ex/ex/vg on HCA, 1.4, and falls within the range of AGS ideal on the HCA. Why did GIA not like it so much? Maybe because it has a slightly larger table.

 

[Garry H (Cut Nut)]

Date: 11/27/2007 1:12:37 AM
Author: milton333
The lab definitions of what is ''ideal'' are based on mathematical formulas for very precise ranges of proportions in an rb. An ''ideal'' rating does not always mean the best light performance, and vice versa. For example, I have my eye on a GIA-cert''d rb that scores only a ''very good'' cut from GIA, but ex/ex/ex/vg on HCA, 1.4, and falls within the range of AGS ideal on the HCA. Why did GIA not like it so much? Maybe because it has a slightly larger table.

GIA did not do an excellent job in developing their cut grade system.
http://journal.pricescope.com/Articles/50/1/Letter-to-the-Editor-of-the-Australian-Gemmologist.aspx gives a rather heavy review of their approach and where a group of my cronies feel they went astray

GIA actually allow a larger table than AGS will (now).

 

[sparkleslee]

i see that the GIA excellent ''box'' is a lot larger than the AGS ideal box.

it seems like my pavilion depth (and therefore, pavilion angle?) is the one throwing it off. if it were just a few 0.x% or 0.xdeg higher, it''d fit into the ideal/excellent cut guidelines.

i''m still not sure what a lower pavilion angle does... is it too shallow a stone? b/c the crown angle and table all seem to be right on.

so confused!

 

[phiberoptik]

I also purchased a GIA Cut=VG diamond and it produced and HCA of 0.6. I even did a personal performance test in the store with an AGS Triple 0 of the same size put next to it side by side and unknowingly picked my GIA VG over the AGS Triple 0 as I thought it performed better.

As I learned in my decision process, looking at the diamond and comparing it made the difference for me.

 

[sparkleslee]

i just realized something...

if crown angle and pavilion have an inverse relationship, and my pavilion is low, does that mean the crown angle is high? would i even be able to tell the difference visually if the pavilion were a smidge higher?