Symmetry, Polish & Proportions
All diamonds submitted to GIA’s Gem Trade Lab for grading will receive a formal grade for symmetry and polish while only 57/58 faceted rounds will receive a cut grade. Here’s what you need to know about Polish, Symmetry and Proportions.
Symmetry is the equality between corresponding parts of a stone. In rounds, the most common symmetry variations are
- misaligned facets
- facets failing to point properly
- misshapen facets
- off-center tables and culets
- out of round girdle outlines
- tables not parallel to girdle planes
- wavy girdles (where the girdle plane resembles a warped record!)
Below are examples of these and other symmetry variations. On most diamond grading reports symmetry is either graded as
- very good
The execution of the design, the precision of the cutting details, and the quality of polish are collectively called "finish".
This is the icing on the cake. For maximum brilliance, sharp scintillation, and clear dispersion, good polish is essential. Diamond will take and keep the best polish of any gem, and good finish allows all of its qualities to shine and sparkle at their very best.
Many of the things that are considered under polish are really clarity characteristics: bearding, scratches, polish lines, nicks and pits. To these we add the condition of the girdle, facets and culet. Since these are very minor differences, the finish can only affect clarity grades such as FL, IF or VVS1. Symmetry and Polish are the only two kinds of cut grades that a GIA report gives you. When it comes to the more important aspect of proportions, you've got something to learn.
While polish and symmetry deal with external characteristics regarding craftsmanship, the actual proportions a diamond is cut to will ultimately determine how that particular diamond handles light. For example you can have a diamond that has the top grades for symmetry and polish yet be an optical disaster if all 57 facets are not cut to the proper proportions.
In many cut grading systems (including GIA’s) proportional data is imperative in determining a final cut grade. The factors considered when looking at the overall proportions are:
- Table %
- Crown Height %
- Crown angles
- Star %
- Upper half angles
- Girdle thickness
- Pavilion Depth %
- Pavilion angles
- Lower half facet length
These proportional measurements are listed for every round brilliant on a GIA Report and looks like this.
On an AGS Report like this.
Before January of 2006 the only conservative lab grading cut quality was AGS and from 1996 till that time utilized a proportion based system for cut grading, looking at only total depth, crown angles, pavilion angles, table size, girdle thickness and culet. With the introduction of light performance analysis tools like ray tracing and reflector based technologies AGS labs abandoned the former system and altered its cut grading system to take into account the measurements and light performance of all 57 individual facets and not just on round brilliants but AGS has went onto include cut grading and light performance analysis on princess cuts, cushion cuts, asscher cuts & emerald cuts as well! As of this publishing GIA is in the works of developing a cut grading system for fancies. For sake of brevity here are 3 profiles of diamonds cut to different sets of proportions. One being shallow, one being steep and one being what is considered ideal. Considering there are 57 facets on average in each cut there are literally zillions of various proportions a diamond can be cut to. As we progress you’ll learn how different proportion sets either help or hurt the beauty of the diamond.
Profile of a shallow cut diamond.
Profile of a steep cut diamond.
Profile of a diamond with Ideal proportions.