The cut of the diamond refers to the precision and expertise of the finished shaped stone. A good cut can enhance a mediocre quality diamond and a bad cut can literally destroy the value of a good stone. Diamonds are cut in facets, or flat planes. Each should be precisely in alignment. Modern diamonds are always faceted to play up their inherent qualities. Other gems can be rounded like a star sapphire, that particular shape is known as cabochon.
The facets reflect light back through the diamond and causes the glitter and rainbow effect so characteristic of diamonds. Beautifully cut diamonds display the most vivid fire with the least loss of brilliance.
The classic round cut has 58 facets. It allows the most light to be reflected back up through the diamond. The face of the diamond is the flat surface you see when looking directly down at a diamond. It is surrounded by 33 facets. The girdle is the rim around the crown of the diamond. The pavilion is the bottom portion of the diamond and has 24 facets plus the tiny facet at the point of the pavilion called a cutlet.
The crown portion of the diamond should measure 1/3 of the depth of the pavilion. Diamonds are always faceted to play up their inherent qualities. Choosing the right cut can enhance a diamond's color (yes diamonds do come in colors) and hide flaws. A poorly cut diamond can be more prone to shearing or shattering.
Diamonds can be cut to look larger by decreasing the pavilion and increasing the crown. In other words the diamond is broad but shallow.
Small diamonds can be cut with only 17 facets and are known as single cuts. Swiss cut can be used for small diamonds as well and have only 33 facets.
There are other cuts for diamonds. The quadrillion is a square cut known as the princess cut. A round diamond can be cut with 144 facets. And a marquise shaped diamond, (think of a football with pointed ends) can have a dream cut.
The cut of a diamond is one of the four factors that determines the value of the stone.