Ratio
This one changes what you see. It is worth spending on.
Length divided by width. For ovals, 1.30 to 1.50 is classic; for cushions, 1.00 to 1.05 is square, 1.15+ is elongated. It changes the look more than the certificate grades do.
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Carat · Certificate · Clarity · Color · Cut · Eye-Clean · Fluorescence · Lab-Grown Diamond · Table and Depth