There are several parameters that determine the grade of a diamond cut: symmetry, proportion, polish quality, and polish.
Symmetry
- Is the culet centered?
- Is the diamond perfectly round?
- Is the table centered?
- Are the facets identical in size and shape?
- Is there alignment between the upper and lower facets?
- Is there alignment between the crown and the table?
In a diamond with good and consistent symmetry, light passes through more effectively, causing it to sparkle more. Symmetry is evaluated and noted by the GIA using the following terms:
- Poor
- Fair
- Good
- Very Good
- Excellent
Proportion The ideal cut was invented in 1919 by a Belgian diamond cutter and mathematician from a family of Jewish diamond cutters, Marcel Tolkowsky. The ideal cut applies to round diamonds with 58 facets.
- Facet = FACET = polished surface. The proportions for a diamond with a diameter of 100%:
- Diamond depth - 59.3%
- Pavilion depth - 43.1%
- Crown height - 16.2%
- Table size - 53%
- Crown angle - 34.5º
- Pavilion angle - 40.75º
The ideal cut is designed to reflect the maximum amount of light, resulting in maximum sparkle. Diamonds display three types of light play:
- Brilliance (Luster/Brilliancy): The white light that returns to the viewer's eye from the stone's surface (usually the table).
- Fire (Fire/Dispersion): The rainbow colors seen in a diamond, created by the dispersion of white light.
- Scintillation: The sparkles of white light returned from the diamond due to the movement of the diamond, the observer, or the light source.
Polish After the diamond is cut to its desired shape, it undergoes polishing to smooth the facets and give them a shiny appearance, allowing light to pass through perfectly. Unlike diamond cleanliness, which refers to its natural internal flaws, polish relates to external flaws that can be altered by a diamond cutter.
External flaws considered when grading polish:
- Burnt (white) surfaces (due to polishing)
- Scratches
- Unpolished girdle
- Chipping
- Knife-edge - an unpolished part of the stone
- Extra facet - an additional facet that should not be present
- Abrasions - caused by diamond rubbing against diamond, typically when stored together without separation
- External growth lines - result from interruptions in the diamond's crystallization process, creating visible lines on the stone
The quality of the polish is determined and noted by the GIA using the following terms (at X10 magnification):
- Poor
- Fair
- Good
- Very Good
- Excellent
