The HCA is coarse filter applied to GIA XXX to narrow the field to those having potential. That potential is confirmed by an IS or ASET, H&A (symmetry), and physical examination (video, in person, etc.). The HCA is not used to select a stone as 2 stones may have the same angles and score the same, but one may have highly variable angles while the other one are tighter leading to less leakage and greater symmetry. HCA is a rejection tool only.
I'm curious about the over-1, under-1 thing because ours has numbers similar to the ones you listed and an HCA of exactly 1.0 which is much closer to Pendants & Necklaces than Best for Rings.
Her ring is stunning and I haven't noticed any problems on her finger at all. But numbers only tell part of the story of course...
Depth % 61.6
Table % 55.9
Crown Angle 34.6
Star 49.0
Pavilion Angle 40.7
Crown % 15.2
Lower Girdle % 78.0
www.whiteflash.com/loose-diamonds/round-cut-loose-diamond-3955177.htm
To answer your question as briefly as possible, the under 1 score is most likely due to the Pavilion Angle of 40.6. It seems that most combinations with a PA of 40.6 return an HCA score under 1 except for when the Crown Angle is 35.5. I believe this is due to the potential obstruction issues (meaning diamond going dark when viewing up close) that can be caused by lower pavilion angles at around 40.5 degrees, combined with the fact that GIA rounds the average of the pavilion angles to the nearest 0.2 degrees. Thus, a 40.6 pavilion angle could actually be closer to an average of 40.51 degrees, and some of the individual pavilion mains may dip below 40.5. This is why some prefer to select only GIA stones with 40.8 Pavilion Angles. Even so, I don't think that a 40.6 pavilion angle in a GIA graded diamond is really ever an issue unless it's combined with a lower crown angle as well (like a 40.6/34.0 combo).
The "HCA under 1 is best for pendants/earrings" argument is based on avoiding these shallow combos like 40.6/34.0 or something like 41.0/32.5 (which can be a very bright combo for earrings/pendants) for stones that will be viewed up close in rings. But not every stone with an HCA score under 1 will suffer from this obstruction. The 40.6/35.0 combo also typically scores under 1, but I've never seen a case on here where someone has reported obstruction in a well cut 40.6/35.0 diamond.
Furthermore, the stone you are considering is an AGS stone, which means that the 40.6 Pavilion is actually an average of 40.6 (or pretty darn close). And with an ACA stone, I wouldn't worry about what the HCA says in this case. A 40.6/34.9 combo is going to be beautiful.
40.5 actual angles and none of them drop below ~40.45 is a totally different stone than one that has 5 of them below ~40.45 in a ring.
Thx for posting that FP
Shallow stones are bad for retailers because when you are buying them you are looking really closely on inspection. Step back and look at them the way you would in yours or a friends ring etc and you may well prefer a slightly shallower stone than a Tolkowsky. The slightly shallower stone measures and looks larger because it has better peripheral light return.
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