Proportions, Symmetry & Polish

As one of the 4Cs, cut is the factor that most influences a diamond’s price — and cut is really three things working together: proportion, symmetry and polish. Any imprecision in proportion, or carelessness in the finish, costs the stone appearance and brilliance.

GIA proportion ranges

The figures below show how a round brilliant’s proportions map to the grade bands, from excellent (1) to below average (4).

Measure	1 — Excellent	2 — Very good	3 — Average	4 — Below avg
Table size	53–60%	61–64%	65–70%	>70%
Crown angle	34–35°	32–34°	30–32°	<30°
Girdle	medium – slightly thin/thick	thin – thick	very thin – very thick	extremely thin/thick
Pavilion depth	43%	42–44%	41–46%	<41% / >46%
Culet	none – medium	fairly large	large	very large
Finish	very good – excellent	good	average	below average

For stones under 0.50 ct the excellent table band widens slightly, to 53–62%.

Reading the finish

Symmetry describes how precisely the facets meet and align — the surest sign of which is a clean Hearts & Arrows pattern. Polish describes the smoothness of each facet’s surface, free of the fine lines and marks left by the wheel. Cut grades run very good · good · medium · poor; certain laboratories add an “excellent” grade, though it remains a debated addition not adopted by all labs or for all cuts.

When a diamond is internally flawless, even its external features — naturals, extra facets — feed back into the grade of the cut. Nothing about a fine stone is truly separate from anything else.