Cut
This one changes what you see. It is worth spending on.
Cut is what makes a diamond sparkle: proportions, symmetry and polish. It matters more to how the stone looks than color or clarity. For rounds, insist on Excellent cut; for fancy shapes, look at the video.
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Carat · Certificate · Clarity · Color · Eye-Clean · Fluorescence · Lab-Grown Diamond · Ratio · Table and Depth