Cut is not the shape, but a diamonds proportions, symmetry and polish. Cut is the most important aspect of a diamonds brilliance and fire. Diamonds must be cut to proper proportions to maximize light going in and coming out of the top (or Table) of the diamond. Diamonds which are cut too deep or shallow, or have very thick girdles will not reflect light properly.
"Well Cut" Diamonds sell at a Premium while "Poor Cut" & "Commercial Cut" Diamonds sell at "Discount" Prices. The Diamond Cuts in the slide show to the Right, except "Well Cut Diamond", are examples of Commercial or Discount Diamonds.
WELL CUT DIAMONDSPROMOTIONAL DIAMONDS IDEAL ←EXCELLENT ←VERY GOOD ←GOODι COMMERCIAL→ FAIR→ MINUTE→ POOR