When polishing a rough diamond the aim is to cut the heaviest, most valuable polished diamond possible. This often means polishing a diamond with imperfect symmetry to avoid inclusions or just to achieve a “magic weight” (like 2.00 carats). Symmetry is an important element of diamond finish. Symmetry refers to the exactness of the shape and arrangement of facets. In the real world, very few diamonds are perfectly symmetrical, but symmetry is less important to the overall beauty of a diamond than the critical facet proportions.
The polished diamond may be slightly off-round, have variations in girdle thickness, tilting of the table, and off-centering the table or the culet. Contrary to popular belief, symmetry defects in a diamond are often the result of a highly skilled cutter rather than an indication of poor cutting skills. To the unaided eye, finish features usually have little effect on appearance. The importance of symmetry is less important in diamonds that have lower clarity grades. Symmetry is more important in diamonds that have very high clarity grades.
Proportion-Related Symmetry Features
- Out-of-round: deviation from the circular shape of a round diamond; a flattened area such as that created by a natural or extra facet also constitutes out-of-round. Comparison of the minimum and maximum diameters can help assess roundness.
- Table off-center: deviation of the table from the central position on the crown; results in opposing bezels of differing sizes. When viewed through the table, more of the pavilion is seen to one side of the culet than the other.
- Culet off-center: deviation of the culet from the central position on the pavilion; results in the cross-line formed by lower half facet junctions to be bowed or bent. When viewed through the table, more of the pavilion is seen to one side of the culet than the other.
- Table/culet alignment: displacement of the table facet and culet in opposite directions.
- Table and girdle, not parallel: the girdle plane is not parallel to the table.
- Wavy girdle: undulating girdle.
- Girdle thickness variation: variation of the girdle thickness at “valley” positions.
- Crown angle variation: all eight crown angles are not equal; typically related to table off-center.
- Pavilion angle variation: all eight pavilion angles are not equal; typically related to culet off-center.
Facet-Related Symmetry Features
- Misalignment: displacement of the crown and pavilion facets in relation to each other.
- Non-Pointing: fully formed facet that does not reach its prescribed location (short facet) or is incompletely finished (open facet), resulting in adjoining facets not meeting at precise points.
- Misshapen Facet: difference in shape or size between one facet and another of the same type; or distortion of a given facet.
- Non-Octagonal Table: the table is not a regular octagon; results in misshapen star and bezel facets.
- Missing Facet: asymmetrically missing or deleted facet.
- Natural: part of the original rough diamond’s surface that remains on the polished diamond; typically causes an out-of-round girdle outline, short facets or misshapen facets.
- Extra Facet: additional facet placed without regard for symmetry and not part of the standard cutting style; typically causes an out-of-round outline, short facets or misshapen facets.
GIA Symmetry Grade System
- Excellent: Ranges from no symmetry features to microscopic features that are difficult to see at 10X magnification.
- Very Good: Minor symmetry features are seen face-up at 10X magnification.
- Good: Noticeable symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance might be affected with the unaided eye.
- Fair: Obvious symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance is often affected with the unaided eye.
- Poor: Prominent symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance is significantly affected when viewed with the unaided eye.
Is it Necessary to Spend More for an Excellent Symmetry Grade?
The performance of a diamond graded Excellent or Very Good Symmetry is far superior to a diamond with a Good Symmetry grading or lower. Generally, diamonds with Excellent Symmetry Grade prices are not that much higher than diamonds graded Good Symmetry. Until the 1980’s symmetry was not paid particular attention. Within the last 20 years, the diamond industry has become more transparent as consumer demands for perfection have increased. Symmetry measurement is a process full of inherent uncertainties, but GIA’s efforts to achieve smaller uncertainties have been successful. Starting in early 2012, GIA will use values in a tighter range to attain greater consistency than is possible through visual assessment alone. This reminds us that as the industry improves their measuring techniques (improved standards) we need to remember to purchase diamonds having current Diamond Grading Reports!
Final Tips on Symmetry:
- Avoid Fair and Poor Symmetry Grades.
- Excellent and Very Good Symmetry Grades are most desirable.
- Excellent Symmetry Grades can be found at reasonable prices relative to all other grades.
- Diamonds having superior Symmetry grades should take precedence over Excellent Polish Grades.
- Always pay particular attention to entries under Keys To Symbols
This article was contributed by Frank Fisher.