What does 'cut' mean for a diamond?

Cut describes two distinct things in diamond grading: the shape a diamond is polished into, and a quality grade assessing how well that polishing was executed. The quality grade, used by IGI for lab-grown diamonds, runs from Excellent down through Very Good, Good, Fair, and Poor. It reflects proportions, symmetry, and polish, which together control how light moves through the stone. Proportions and symmetry are the primary drivers of brilliance and fire, not the outline shape itself. Shape does affect the character of that light return: a round brilliant produces a uniform, concentric sparkle, while a princess cut delivers a more angular, directional flash. When comparing lab-grown diamonds, two stones of identical carat and color can look dramatically different if their cut grades diverge. Prioritize Excellent or Very Good cut grades before adjusting other variables, since no color or clarity upgrade compensates for light lost to poor proportions.