Grading the symmetry of round brilliant cut diamonds at GIA (USA)

The Gemological Institute of America (GIA) also divides finishing grading into symmetry grading and polishing grading (GIA Diamond Grading, 1993). When a symmetry distortion affects the appearance of the diamond or indicates some special efforts of the manufacturer willing to save the diamond weight, such a distortion is considered as a proportioning flaw. The noticed major symmetry distortions are listed in that worksheet area meant for proportioning (which is not graded), while minor ones are referred to finishing. Main symmetry distortions that may be found in a round brilliant cut diamond include both those always considered minor and those which, under certain conditions, can be considered major. These distortions may be of the following types:
- off-center culet;
- off-center table;
- misshapen facets;
- the table is not a regular octagon;
- non-parallelism of the table and the girdle;
- out-of-round girdle outline;
- misalignment of crown and pavilion facets;
- facets not properly pointed;
- wavy girdle;
- additional facets (naturals and extra facets).

The symmetry is tested first with a 10x loupe, then with the naked eye. All the symmetry distortions, which evidently affect the appearance of the stone, are considered major. The more frequent of these are the following:
- table or culet noticeably off-center, observed through a 10x loupe;
- girdle outline out-of-round, observed with the unaided eye;
- evident non-parallelism of the table and the girdle or evident waviness of the girdle, observed through a 10x loupe;
There are the following symmetry grades: “Excellent”, “Very good”, “Good”, “Fair” and “Poor”. However, such a grade concerns only the minor symmetry distortions, so the major ones should not be graded or divided into any classes. Nevertheless, if a diamond has at least one major symmetry distortion, its symmetry can be graded only as “Fair” or “Poor”.