The symmetry of a diamond refers to the symmetrical alignment of facets, and plays an important role in the cut grading. Like cut, some gemmology labs start at Ideal, others Excellent. Fancy shaped diamonds have a symmetry grading.
Polish influences how well light is able to pass through a diamond and is very important to a diamond's brilliance. It is necessary to select a diamond that is laboratory certified with good, very good or excellent polish (some labs also grade “Ideal”). Diamonds that have poor to extremely poor polish are less brilliant because they have microscopic polish lines that blur the surface of the diamond. These polish lines reduce the amount of light that enters or exits a diamond.
Diamond Symmetry
When a faceted diamond is observed under normal x10 magnification, one can easily study the symmetry aspects of the cut. The symmetry of a faceted diamond plays a substantial role in the cut evaluation of diamonds.
There are certain factors that have to be checked for the perfect cut grade:
Symmetry of the sides of the table edges
The table facet of the round brilliant cut should form a perfect octagonal shape, sometimes one or more blunt table edges are observed. There should be four sets of parallel sides of the octagonal shaped table. When the edges are not straight but wavy, a good symmetrical cut is ruled out.
Centricity and parallelism of table
The table facet has to be parallel to the girdle plane so that the crown height is equal in all directions. Many a time the table is tilted to remove some inclusion near the surface of the table. The octagonal shape of the table should have its centre exactly above the culet, otherwise it is called eccentric. An eccentric table creates different sizes of crown facets which affects the brilliance of the diamond.
Centricity of the culet
The fine point or the smallest facet at the base of the brilliant cut is the culet. Sometimes the culet is faceted. This culet should lie in the centre of the octagonal table. It is very easy to see whether the culet is eccentric from a side view of the stone as well as on looking through the table. If the culet is off-centre then it indicates that the pavilion facet angles are unequal. This gives a very adverse effect on the reflection of light symmetry.
Sharpness in the intersection of the facet edges
Excentric culetIn a perfect cut, the edges of individual facets must meet in a single intersection without any truncated ends. Truncated facets do not meet at a point and affect the uniformity of the cut. A truncated main facet when observed through the table interrupts the symmetrical effect of the adjoining facets.
Alignment of the crown and the pavilion facets
The kite facets of the crown should exactly intersect with the pointed edge of the pavilion main facet and the upper girdle facet should synchronise with the girdle facets. There should be a perfect alignment and no sideways displacement with respect to each other. As this clearly affects the regularity of the cut and gives rise to uneven girdle as well as reduced brilliance.
Diamond Polish
Polish influences how well light is able to pass through a diamond and is very important to a diamond's brilliance. It is necessary to select a diamond that is laboratory certified with good, very good or excellent polish. Diamonds that have poor to extremely poor polish are less brilliant because they have microscopic polish lines that blur the surface of the diamond. These polish lines reduce the amount of light that enters or exits a diamond. When selecting your diamond be aware many diamonds possess poor polish. Diamond cutters can greatly reduce labor costs by not taking time to properly polish a diamond.
Polish terminology as seen on grading certificates:
P = Poor
Prominent heavy polish features are seen face-up at x10 magnification.The luster of the diamond is significantly affected when viewed with the unaided eye.
F = Fair
Obvious heavy polish features are seen face-up at x10 magnification.The luster of the diamond is affected when viewed with the unaided eye.
G or GO or GD = Good
Noticeable polish features are seen face-up at x10 magnification.The luster of the diamond may be affected when viewed with the unaided eye.
VG or VE = Very Good
Minor polish features are seen face-up at x10 magnification.
E or X or EX = Excellent
Ranges from no polish features to a few minute polish features that can be viewed with difficulty face-up at x10 magnification.