By Ashley Bailey
, Tuesday, July 25, 2006
It’s important for buyers to know that aspects of the 2006 GIA diamond cut grading system are skewed to favor mass manufacturers over consumers.
GIA performed studies for 15 years then enlisted human diamond observations. Problems arose when only 58% of the human results correlated to their prior research: Those observations were made primarily by trade people and demonstrate a bias towards mass manufacture and mass sales. GIA adapted their prior research to accommodate the trade observations. The result is a system which favors mass manufacturers over consumers.
Are all GIA EX diamonds bad?
The system uses external measurements (proportions) to predict how well a diamond will sparkle and reflect light internally. Some proportions sets agree with proven systems.
What should the diamond buyer beware?
(1) The width of the EX grade, (2) abundant steep/deep combinations. Also, be aware of the ‘rounding’ that occurs on GIA diamond grading reports; the angles you see are not reported to the tenth of a degree like AGS reports or Sarin/Helium scans. There are other issues, but these are the main ones to look out for when considering a GIA ‘EX.’
I. GIA EX GRADE IS VAST, MANY WILL BE EQUIVALENT TO AGS4
Using the proven AGS 11-level system for comparison, GIA’s top grade is extremely wide. Alone, it spans the top 5 AGS grades. A diamond graded GIA Excellent may be an AGS4 as easily as an AGS0. Further, each lower AGS grade leads to extra weight within the same GIA grade. Mass manufacturers will cut the heaviest possible GIA Excellent. This means when a consumer is buying a GIA Excellent the statistical chance of him buying an AGS4 will be high. GIA may tell you that within their top grade there is no visible difference but AGS will tell you it can be divided into 5 different grades.
EXAMPLE A: Wide EX grade
II. ABUNDANT STEEP/DEEP COMBOS ALLOWED, REDUCING PERFORMANCE & SPREAD
Steep/deep is positive for manufacturers but negative for buyers. As the diamond is made steeper weight is added and spread is reduced, so the diamond looks smaller than it should for its carat weight. Steep/deep combos also leak light so performance is reduced. Very steep/deep entraps body color, so a diamond graded F may actually appear like a G or H.
EXAMPLE B: Light leakage and reduced performance
GIA Excellent - the same diamond as in Example A. There is enough light leakage that the holder’s finger can be seen through the table of the stone.
Actual IS, simulated IS and cut quality estimate (below)
From dmc file: 56.8t 62.9d 35.28c 41.44p dmc file
GIA Sarins should have rounded to 41.4 and 35.5 rather than 41.6 and 35 (?)