Cut is not shape — it's the precision of the faceting, and it controls the one thing everyone actually sees: sparkle. A perfectly cut diamond returns light to your eye; a poor one leaks it out the bottom.
The grades
Ideal/Excellent — light return at the physical maximum; the only grades we recommend for rounds. Very Good — 5–10% cheaper, difference visible mostly to instruments. Good and below — visibly dimmer; the discount isn't worth it.
For fancy shapes (oval, pear, cushion) labs grade cut less strictly, so proportions and video matter more: look for even sparkle across the stone with no dead zones or heavy bow-ties.
The numbers that matter
On a round: depth 59–62.5%, table 54–60%, and symmetry/polish of Very Good or better. Inside those windows, trust your eye over the decimals.