Diamond proportion refers to the relationship between the size, shape, and angle of each facet of a diamond. A wide range of combinations are possible, ultimately determining the diamond's interaction with light.

Of all the 4Cs, Cut has the greatest effect on a diamond’s beauty. In determining the quality of the cut, the diamond grader evaluates the cutter’s skill in the fashioning of the diamond. The more precise the cut, the more captivating the diamond is to the eye.
Cut does not refer to the shape of the stone.

If the diamond cut is too shallow, entering light strikes the pavilion facet at a low angle and passes through the facet (refracts), escaping through the bottom of the diamond.

If the diamond cut is too deep, entering light strikes the first pavilion facet at an angle sharp enough to reflect to the second pavilion. But the light strikes the second pavilion at too low an angle, causing the light to refract (pass through the facet), escaping through the bottom of the diamond.

In a ideal cut diamond, the light strikes each pavilion facet at an angle which allows most of the light to reflect back to the crown (top). As it passes through the crown facets at a low angle, the light refracts upon exit. In this case, refraction is a good thing, as the bent light travels to the observer's eye and is perceived as a lively fire.

Excellent + Very Good
These diamonds will have a high degree of brilliance, fire and scintillation

This grade will generally be a bit darker or lacking scintillation

Diamond in this category lack brightness, fire and scintillation

Poor (Very Shallow cut)
Diamonds in this category show very littlebrightness, fire and scintillation

diamond cut