Many people don't realize the Symmetry grade on a diamond's laboratory report only refers to external meet point symmetry (facet junctions meeting equally) and uniformity of the diamond's external shape.
Optical Symmetry, on the other hand, is the type of symmetry that is seen through a Hearts & Arrows viewer (both crown and pavilion views) or an ideal-scope/ASET image (crown view). Optical Symmetry is the observable result of how the diamond's facets physically align with each other internally, and how light reflects inside the diamond.
The fact that some diamond experts use simply 'symmetry' when referring to optical symmetry confuses the issue for newcomers.
Graphic examples follow.
(they are not the same)
EXAMPLE A: Meet Point Symmetry (graded by labs)
What is evaluated here?
Facet junctions meeting equally: Seeing if the lines are clean and there is no misalignment.
Uniformity of the crown in relation to the pavilion: Making certain all of the main facets are aligned - that there is no “twist.”
The diamond's symmetry grade you see on a laboratory 'certificate' is an assessment of the above. Studies have concluded that there is negligible visible difference between Ideal, EX and VG in lab-issued symmetry and polish grades.
EXAMPLE B: Optical symmetry (not graded by labs)
Also called Patterning or Hearts & Arrows Patterning
What is evaluated here ?
Good physical symmetry resulting from a detailed indexing of the facets (crown and pavilion facets appear in their correct place on the ‘clock’).
How precisely patterned and flat facets are relative to their opposites (that there is no shift in the azimuth). When a cutter takes the facet (polishes it) from E to W, rather than from N to S, it can cause an azimuth shift, resulting in facet yaw. This will show in optical symmetry.
Variation between crown and pavilion angles: It is acceptable to have some variation (subject to limitations) as long as there is no E-W yaw and opposite facets average correctly. Some might consider this an angles/proportions issue, but it is directly related to physical symmetry, as the question here is opposite facet relations, averaging and resultant light interaction. For that matter, it has been observed that a slight variation in opposite angles can improve overall scintillation and contrast as long as there is no presence of E-W yaw.
What does this mean?
Meet point symmetry is external, the labs do not look at how each individual facet performs or interacts with other facets internally; They check only junctions and crown/pavilion relations. This means a diamond can receive a grade of Ideal or Excellent in symmetry but may still have indexing issues (bottom clock in relation to the top clock), facet tilt, or yaw and extreme variations in angle measurements. The only way to observe physical symmetry is through a reflecting viewer.
EXAMPLE C: Both of these diamonds were graded EXCELLENT in Symmetry
Both diamonds were photographed using a hearts & arrows viewer. Both received a top grade of Excellent in symmetry. The difference in facet relationships is obvious, but on paper the symmetry grade is the same.
This is how patterning - or optical symmetry - differs from lab graded symmetry. To learn more about optical symmetry, visit our information on Hearts & Arrows and our Whiteflash A Cut Above superideal Hearts & Arrows diamonds..
For more specific questions ask our experts