The shape of your gemstone is defined by its cut. By polishing small flat facets at planned regular angles, rough stones are cut into precise shapes which reveals the beauty of the gem.
Diamonds are most often cut into a shape called round brilliant, but there are many other cuts available, the main types of which are princess, marquise, heart, emerald and oval. Other typical cuts can include pear, baguette, trillion and Asscher.
Some stones such as opal and turquoise are cut and polished into smooth dome-shaped gems, known as the cabouchon cut.
A well-proportioned cut with a fine finish will maximise a stone's beauty. When all other factors are even (carat, colour and clarity), a better cut gem will be more valuable.
The main types of diamond and gemstone cuts are illustrated below. Note that diamonds tend to have more facets than other types of gemstones.