CUT

A diamond that is masterfully cut displays maximum brilliance and fire. Cut and proportion are extremely important
because they have the greatest influence on the sparkle and value of a stone. A diamond that is well cut reflects light from one facet to another and disperses it back through the top of the stone.

Cut also refers to shape — round, square, 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.