Today, the GIA measures all of the symmetry components by an automatic device that measures parts of the symmetry that the human eye cannot notice at all.
There are two different types of symmetry:
- Is the culet in the center of the stone?
We can imagine radii from the culet to the girdle and check that they are equal in their lengths. Or we can check if the culet divides the table and the star facets into 4 equal squares, and confirm that the cross is straight.
2. Is the table in the center of the stone?
We can imagine lines from the table to the girdle and see if they are equal in their lengths.
3. Is the girdle round?
We can trace the girdle visually in the Loupe and see if it is round. It is important to pay attention to naturals/chips/extra facets in the girdle area that may affect the roundness of the girdle.
We can check using a gauge. If there is a deviation of more than 1.5% between the minimum diameter to the maximum diameter, the girdle is not round.
4. Is the crown height equal? We should check it in the profile of the stone.
- Is the table a regular octagon?
- Are all the facets (of the same type) equal in their size and shape?
- Are the intersections of facets correct and uniform?
- Is the crown even with the pavilion? Do the upper girdle facets line up with the lower girdle facets and the upper main facets line up with the lower main facets?
- Do the upper and lower main facets close in points on the girdle?
- Are there large and non-symmetric extra facets and naturals?
- Does the girdle have extreme changes in its width?
Rules for determining the symmetry grade
- If the structure symmetry and the facet symmetry are perfect, the grade will be: Excellent.
- If there are very slight deviations in the structure symmetry and in the facets symmetry the grade will be: Very Good.
- If there are small deviations in the structure symmetry and clear deviations in the facet symmetry the grade will be: Good.
- If there is a very strong deviation in the structure symmetry (such as: off-center culet or table, girdle not being round, crown height not equal), the grade will be: Fair.