Refers to the angles and proportions of a diamond.
Based on scientific formulas, a well-cut diamond will internally reflect light from one mirror-like facet to another and disperse and reflect it through the top of the stone. This results in a display of brilliance and fire, thereby placing well-cut diamonds higher on the Diamond Quality Pyramid than deep or shallow-cut diamonds. Diamonds that are cut too deep or too shallow lose or leak light through the side or bottom, resulting in less brilliance and ultimately, value.
Cut also refers to shape—round, princess, pear, or heart for example. Since a round diamond is symmetrical and capable of reflecting nearly all the light that enters, it is the most brilliant of all diamond shapes and follows specific proportional guidelines.
Non-round shapes, also known as “fancy shapes,” will have their own guidelines to be considered well-cut.