Perplex by AGS0 Table %

[Melis]

Hi Everyone,

I've read in many places that AGS 0 has to have a table size of 52.4-57.5%. I'm a bit perplex because I'm looking at a round brilliant that has a table of 57.6% and yet it's graded AGS0. According to all I've read, a 57.6% table would be an AGS 1. Has this table scale changed to include larger table size or could it be an incorrect grading on the AGS report?


Table proportions AGS cut grade

46.3% and smaller AGS (10)
46.4% to 47.3% AGS (9)
47.4% to 48.3% AGS (
48.4% to 49.3% AGS (7)
49.4% to 50.3% AGS (5)
50.4% to 51.3% AGS (3)
51.4% to 52.3% AGS (1)
52.4% to 57.5% AGS (0)
57.6% to 59.5% AGS (1)
59.6% to 61.5% AGS (2)
61.6% to 63.5% AGS (3)
63.6% to 65.5% AGS (4)

 

[JulieN]

This is the old proportion-based cut grading system. New light performance system has much wider table range.

 

[John P]

This is the old proportion-based cut grading system. New light performance system has much wider table range.

Adding to this:

While the possible table range is wider, the overall metric is stricter. The old system was "boxed," where a range was given and any table in the T-range (53-57.5) could pair with any crown angle in the CA-range (33.7-35.8 ) and any pavilion angle in the PA-range (40.5-41.5).

But the outer edges of those ranges did not work well together. For example, 57.5T 33.7CA 40.5CA creates a shallowish combo that would now receive a reduction; becoming a predicted AGS2-3 (also GIA VG) due to lower dispersion and reduced brightness in close viewing. At the other side, 53T 35.8CA 41.5PA creates a deep combo that is now predicted at AGS5 (GIA VG) due to light leakage.

The current system (launched in 2005) now considers all proportions combined. Formerly diamonds with tables smaller than 53 and larger than 58 were doomed. Now a cutter with a suitable piece of makeable rough could cut 48T 34.0PA 41.0PA and it will be a candidate for AGS0... The "bullseye" of AGS0 for 48T is significantly smaller than more practical (53-5 table sizes, but it's out there if a rogue cutter wanted to shoot for it. [Sidebar1; there are practical reasons why manufacturers don't produce table sizes that small, as such a plan is typically not profitable. ] On the other side, diamonds with 60-61T can now earn AGS0. But there, again, the "bullseye" a cutter needs to hit is smaller than the window of possible complimentary angles for 53-58. Nevertheless, there are now some amazing "60-60" makes out there which are robustly AGS0, with a different aesthetic than smaller tabled diamonds. [Sidebar2; I am shocked by the number of jewelers who still aren't aware that 60-60s can now earn Ideal with AGSL... it's been nearly 10 years now, friends.]

Also key: The new (2005) AGS light performance metric isn't chart-based like its predecessor. Instead, the diamond is computer-modeled and ray-traced. In this manner the way all 57 facets work together is considered, rather than just T CA PA averages correlated to lookup charts. That's why I say "predicted" in my second paragraph above: AGSL provides us with cutting guidelines for the broad T CA PA strokes, but if the 40 minor facets are not complimentary, or the diamond is cut inconsistently, or the indexing of the half-facets is irregular, the diamond may still be penalized.

The result is a wider range of tables, but a thinner list of complimentary angle sets which follow the main cutter's line. The widening of the table range has permitted a variety of appearances: 48T 34.0CA 41.0 PA looks much different than a 61T 32.5CA 41.0PA. Yet both meet the "minimum" values AGSL requires in terms of brightness, dispersion, contrast and non-leakage. For cut-focused pros and enthusiasts this is both a joy and an advisory, as all AGS0s do not have the same aesthetic, and some of the "minimum" values may be a bit low - in one category or another - for the seriously pedantic.

I didn't mean to type so much, but I'm enjoying in-flight wifi right now, and the flight attendant keeps refilling my coffee.
#wired

(with apologies to those who hate non-tweety hashtags)

 

[Melis]

Thank you Julie & John! That clears up a lot.

I really enjoyed reading your detail post John and feel honored to have such a pro like you respond to my thread . That was so informative!

i hope you enjoyed your flight and are heading to a nice destination.