A diamond’s “certificate” is its grading and quality report. You should only purchase diamonds with a grading report from an independent, reputable lab. Grading reports list the following:
- Report number – Laser inscribed on the girdle of many higher quality diamonds
- Diamond weight in carats
- Shape and style – Round brilliant cut, Square modified brilliant for a Princess
- Measurements in mm
- Clarity - How visible and how many inclusions: F, IF, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, I3
- Color - How colorless/white: DEF GHI JKL MNO....Z
- Polish quality – How well the diamond was finished and the facets polished
- Symmetry – How symmetrical is the diamond, how close is a round diamond to a round and a princess to a square.
- Fluorescence – Does the diamond fluorescence in ultra violet light, if so how much and what color.
- Inclusion Plots & Angles - On larger higher end reports/diamonds
- Angles - of the diamond in percentage and degrees
- Plot – A plot of the diamond is give showing its inclusions
Knowing which lab issued that report is important, since diamond grading is not standardized or regulated. For example; a diamond reported as F in color at a known “soft” lab might receive a H, or lower at a more reputable lab. The diamond trade discounts diamonds up to 40% based their certificate. To maximize revenue many diamond manufacturers send average diamonds to labs with soft standards, higher quality diamonds to stricter labs and a fraction of their most elite production to be judged by labs that enforce the highest standards.
AGS and GIA Grading Reports
These American organizations were both founded by Robert Shipley in the 1930s and undisputedly employ the most strict and consistent standards in the world. Diamonds with reports from the AGS or GIA command a higher premium and hold their value better than diamonds from softer labs. The world’s best diamonds are typically sent to these labs.
The Gemological Institute of America (GIA) has the strongest worldwide reputation for independence and consistency. The world’s largest and most valuable diamonds have been sent there for decades thanks to their constant color and clarity strictness. Their cut grading system, introduced in 2006, relies on a two-dimensional proportions system based on human observation studies. Their top grade of “Excellent” is in large agreement with the AGS system, though it is far wider and ranges into a steep/deep area that does not result in high performance. Approximately the best 15% of round diamonds will earn the top grade in the GIA system. Many reputable sellers carry diamonds graded by the GIA. The GIA does not yet grade cut in fancy shapes.
The American Gem Society (AGS) is the world’s elite cut grading laboratory. Smaller in scope than the GIA, they enforce the same color and clarity standards while focusing more strictly on cut craftsmanship and scientific light performance. From 1996-2005 they employed the only cut-grading system for round diamonds. That two-dimensional system was replaced in 2005 with a three-dimensional light performance metric that works for several diamond shapes including round and princess cut.
The American Gem Society (AGS) boasts the only cut grading system recognized by the science community. Published by SPIE, the International Society for Optical Engineering, theirs is a diamond-specific metric which evaluates angular spectrum in three-dimensions and takes different distances and tilt into account. Fewer than 1 in 20 round diamonds are capable of earning the AGS Ideal grade and this number falls to 1 in 100 for princess cuts. Only 2% of diamonds are sent to the AGS for grading, so diamonds graded there are rare. Diamonds that cannot succeed there are sent elsewhere.