Academic question-HCA scores vs. cut/polish/symmetry

[mrs taylor]

I have a question about how much weight to put on the HCA vs. the GIA report.

This question is more for if you are unable to see two diamonds. Lets say after looking you have narrowed the choices down to diamond A. vs. diamond B.

Both stones are the same size, color and clarity.

Stone A. has a GIA report that shows it as having x/x/x for cut, symmetry and polish. However when the numbers are plugged in it scores 2.8 on the HCA.

Stone B. has a GIA report that shows it as having an x for cut, vg/vg for symmetry and polish, but it scores 1.3 on the HCA.

Which is the better choice and why? How much stock do you put in the HCA as a tool vs. that GIA cert when buying on the internet?

 

[TC1987]

That's a complicated issue, because the make of the diamond and the table, depth, crown + pavilion angle combo, and the minor facets all affect the indivudual character of the diamond. I like fatter arrows, so I would out probably 76-77% lgf, but someone who likes a more "slivery" look might pick 80% lgf or a Solasfera. People who like broad flashes and chunky facets might pick OEC or OMC. I'd use the HCA as a first-pass elimination tool on a modern H&A RB. But when I'm looking at oddball RB diamonds that are older stones, I'd let my eyes and ASET scope and H&A viewer and loupe decide.

minor facets:
http://www.goodoldgold.com/Articles/MinorFacets/

Todd Gray's 15 second course on H&As: http://niceice.com/diamond-buying-secrets/15-seconds-diamond-buying-success/

eta: I think there have been diamonds shown here on PS where the difference between VG/VG and Ex/Ex is nuance that is not immediately evident to most consumers, and/or might be difficult to notice except in perhaps a direct side by side comparison. People have asked that question about VG/VG before and the answer depends upon how narrowly it missed Ex/Ex. Near miss = it has more potential to be a good performer at a lower price.

 

[mrs taylor]

Yes, it seems that the HCA only really works for modern RB's if I'm reading correctly. I am NOT a numbers gal, but this is completely fascinating to me.

That's interesting to know about the nuance, and it makes complete sense.

So assuming that we're talking nuances, all things being equal is it as cut and dry as "pick the one the HCA says is a better performer"?

 

[Gypsy]

It is a more complicated question, but the simple answer is... HCA trumps as a REJECTION tool (because that is what it is). vg for symmetry and polish is fine. They do not disqualify a stone from contention. They do not trump the HCA. Symmetry doesn't refer to optical symmetry, it's about how the facets connect together. Polish and symmetry do not determine light return and light return is what you should be shopping for. A score of 2.8 does disqualify a stone from contention though on the HCA.

So if you are talking about using one over the other as a more accurate rejection tool, then the HCA score matters more than the polish/symmetry.

That said, neither is a selection tool. Okay? Only idealscopes are selection tools. And ASETs and lab reports from AGS that determine light performance.

HCA is a rough tool. polish/symmetry are just good information. Neither of them is something that you select a stone with.

 

[mrs taylor]

It is a more complicated question, but the simple answer is... HCA trumps as a REJECTION tool (because that is what it is). vg for symmetry and polish is fine. They do not disqualify a stone from contention. They do not trump the HCA. Symmetry doesn't refer to optical symmetry, it's about how the facets connect together. Polish and symmetry do not determine light return and light return is what you should be shopping for. A score of 2.8 does disqualify a stone from contention though on the HCA.

So if you are talking about using one over the other as a more accurate rejection tool, then the HCA score matters more than the polish/symmetry.

That said, neither is a selection tool. Okay? Only idealscopes are selection tools. And ASETs and lab reports from AGS that determine light performance.

HCA is a rough tool. polish/symmetry are just good information. Neither of them is something that you select a stone with.

Extremely helpful. Thank you! There's so much to learn-it's completely fascinating.