The cut (make) of a diamond is a most important feature. Cut is not a reference to the shape of the stone. It refers to the angles and proportions created by the skilled craftsperson while transforming a rough diamond into a polished diamond. A diamond may have the highest colour and clarity, but a poor cut will result in a dulled brilliance.
A well-proportioned diamond follows strict mathematical proportions. The light entering the diamond from the top will be internally reflected from one facet to another before refracting back through the top, & returned to the eye of the observer, giving the stone maximum brilliance, scintillation and fire.
A diamond can be cut to retain maximum weight and to create the illusion of size, but this results in a loss of brilliance. If cut too deep, light will escape through the bottom facets and the centre of the diamond will appear dark. If the stone is cut too shallow, the light will also escape through the bottom facets, giving the diamond a watery or glassy appearance.
A well-cut diamond will internally reflect light from one facet to another and disperse and reflect it through the top of the stone, maximising brilliance as well as fire of the stone.