GIA cut grade & HCA

[missyminx]

I tried searching PS for a thread that may have discussed this before but couldn''t find one similar... while looking through some pears and rounds I found two RBs that did quite well on the HCA.

Stone 1
Cut Grade: Very Good
Polish: Excellent
Symmetry: Excellent
Depth: 60.8%
Table: 57.0%
Crown Angle: 34.5 deg
Pavilion Angle: 40.6 deg
Girdle: Thin to Med, faceted
Star Length: 60%
Lower Half: 80%
HCA rating: 0.7 EX/EX/EX/VG

Stone 2
Cut Grade: Very Good
Polish: Excellent
Symmetry: Excellent
Depth: 60.2%
Table: 57.0%
Crown Angle: 33.5 deg
Pavilion Angle: 40.6 deg
Girdle: Med, Faceted
Star Length: 50%
Lower Half: 80%
HCA rating: 0.7 EX/EX/EX/EX

In my noob opinion, I thought the proportions on these two stones looked quite good, except for the slightly shallow crown angle of stone 2. Please feel free to correct me.

But onto the grading, from what I''ve read here, it seems the GIA grading scale has been somewhat wide for their excellent cut grade rating, thus a very good rating may mean a far more inferior stone as compared to a superideal.

So I''ld like to know if this also means that these stones may not perform as well as a GIA Triple EX stone of excellent proportions? In an instance where you see good proportion numbers, should one check out the GIA ratings FIRST to eliminate, then key in the numbers through the HCA, or does HCA matter more than the GIA/AGS ratings?

P.S. Pears are not an easy bunch to suss out!

 

[Lorelei]

Date: 10/4/2008 4:03:12 AM
Author:missyminx
I tried searching PS for a thread that may have discussed this before but couldn't find one similar... while looking through some pears and rounds I found two RBs that did quite well on the HCA.

Stone 1
Cut Grade: Very Good
Polish: Excellent
Symmetry: Excellent
Depth: 60.8%
Table: 57.0%
Crown Angle: 34.5 deg
Pavilion Angle: 40.6 deg
Girdle: Thin to Med, faceted
Star Length: 60%
Lower Half: 80%
HCA rating: 0.7 EX/EX/EX/VG

Stone 2
Cut Grade: Very Good
Polish: Excellent
Symmetry: Excellent
Depth: 60.2%
Table: 57.0%
Crown Angle: 33.5 deg
Pavilion Angle: 40.6 deg
Girdle: Med, Faceted
Star Length: 50%
Lower Half: 80%
HCA rating: 0.7 EX/EX/EX/EX

In my noob opinion, I thought the proportions on these two stones looked quite good, except for the slightly shallow crown angle of stone 2. Please feel free to correct me.

But onto the grading, from what I've read here, it seems the GIA grading scale has been somewhat wide for their excellent cut grade rating, thus a very good rating may mean a far more inferior stone as compared to a superideal.

So I'ld like to know if this also means that these stones may not perform as well as a GIA Triple EX stone of excellent proportions? In an instance where you see good proportion numbers, should one check out the GIA ratings FIRST to eliminate, then key in the numbers through the HCA, or does HCA matter more than the GIA/AGS ratings?

P.S. Pears are not an easy bunch to suss out!

The diamonds look great and the crown angle is actually fine and not really shallow - it would be preferable with that pavilion angle to have the crown a bit steeper, but an Idealscope would be good to evaluate the diamond. GIA cut grades, however a VG stone can sometimes have good proportions! So evaluate each diamond on its own physical and visual merits.

How I approach it is with GIA EX/ VG, evaluate each diamond on its proportions and sometimes use the HCA. Then once you find the best ones, request Idealscope images to confirm the numbers.

Pears are a completely different kettle of fish!

 

[stone-cold11]

You have to read carefully what how to use the HCA. In the explanation graph, it shows different, rough regions, such as younger people ring, older people ring.

If you look carefully, all the AGS0 rating falls within the young people's ring region. That region is for people with good or short eye sight and would like to see the stone close up, leading to obstruction of light source. The stones that falls into these regions will still perform well with this kind of obstruction. But this is not the case for the older people ring stone region which will show a significant darkening of the stone with this kind of light obstruction.

Stone 2 falls into the older people ring stone region, so it is better to keep people viewing the stone from afar. It is thus, better to use that kind of stone for a ear-ring or pendant as they will not get the same kind of close scrutiny as a ring stone.

Moral of the story? Look at poor cut grading in the low crown and pavilion angles for ear-ring and pendant stones to get a great deal, as poor cut grading will result in a lower price...

EDT:

Stone 1 I am guess is cause by the rounding of GIA numbers given in the report. It should be a GIA excellent cut if those are the actual numbers of the stone.

 

[John P]

To expand on Lorelei's input, the GIA EX range is centered a bit steeper than other systems. There is a lot of agreed overap from the more accepted systems but they do have differences.

For example, in the AGS metric stone 1 is on the border of AGS0 - AGS1 and stone 2 is on the border of AGS1 - AGS2. The diamonds' cut precision and how all 57 facets work with each other would determine the actual grade there (some indications of this would be revealed with an Ideal-Scope or ASET image as Lorelei suggests). In general AGS is a bit more closely aligned with the HCA - although the HCA favors a bit shallower still.

If you're interested, here is an example of how a few different systems (AGS, HCA, AGA and GIA) ‘overlap’ – and don’t: Pavilion angle is what many consider the most critical element. It's usually good for showing where the ‘center’ of different systems... For 53-58% tables, you will find that pavilion angles 40.6-41.0 have the greatest approval cross-system. The AGS cut guidelines suggest most 0 candidates right in that range. The HCA and AGA prefer the shallow side and a bit lower. The GIA system prefers the steep side and higher.

 

[Lorelei]

Bleh - I really need to drink more coffee first thing, what I meant to say was...

The diamonds look great and the crown angle is actually fine and not really shallow - it would be preferable with that pavilion angle to have the crown a bit steeper, but an Idealscope would be good to evaluate the diamond. GIA cut grades can allow for some less desirable combos with both Ex and VG, however a VG stone can sometimes have good proportions and be a worthy stone, so evaluate each diamond on its own physical and visual merits regardless of the lab given cut grade.

 

[honey22]

I am not sure if I understood your first post properly, but I just wanted to note that the HCA is for round stones only, you can''t use it for pears?! You probably knew that, but thought I would chime in just in case

 

[missyminx]

hey honey22, yeah I didn''t use the HCA on the pears... otherwise it might be even more mind-boggling now. I just think it''s hard to suss out a pear based on L/W ratio and depth/table percentage cos there''s no proper formula to calculate the angles like what''s done with the RBs.

dear Lorelei and JohnPollard, when I spot a similar close to excellent stone, I''ll be using the HCA/IS/ASET. It is interesting to find that something can still perform well even though it''s out of the triple excellent range. Like finding a diamond in the rough, so to speak! I never knew the pavilion and crown angles contribute more towards performance than table/depth.

stone-cold11 it''s great that you should mention the older people''s stone vs younger people''s stone. I have not read the fine print under the HCA graph (too fixated with the HCA score), and didn''t realise this. This is unexpected good news, as I purchased recently a stone with with same crown angle that''s being set into a pendant.

My pendant stone specs are:
Depth: 61%
Table: 57%
Crown Angle: 33.5%
Pavilion Angle: 40.8%

I should be getting the pendant early this week, and will get to see this phenomena of old vs young.