Clarity
Beyond a point, this one changes the price more than the stone. It is where most people overspend.
How free the stone is from internal marks (inclusions). From FL (flawless) to I3. VS1 and VS2 stones are almost always eye-clean; paying for VVS or IF buys a certificate grade you cannot see.
More terms
Carat · Certificate · Color · Cut · Eye-Clean · Fluorescence · Lab-Grown Diamond · Ratio · Table and Depth