The angles and finish of any diamond are what determine its ability to handle light, which leads to brilliance.
The Proportions of Diamonds
As shown in the images below, when a diamond is well- cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer’s eye.
This light is the brilliance we mentioned, and it’s this flashing effect that makes diamonds so mesmerizing. In a poorly cut diamond, the light that enters through the table reaches the facets and then ’leaks’ out from the sides or bottom of the diamond rather than reflecting back to the eye. Less light reflected back to the eye means less brilliance.
 |
 |
 |
| Shallow cut diamond | Well cut diamond | Deep cut diamond |
| Light escapes trough the paviilion before it can be reflected. |
Light is reflected from one facet to another and then dispersed through the top of the stone . |
Light escapes trough the opposite side of the pavilion. |
The GIA standards for Ideal - Excellent cut are describe below.
The gradation for the proportions of a diamond is Excellent, Very Good, Good, Average, Poor.
Diamond Symmetry
Symmetry measures how well the facets match the other facets. A diamond cut with very good or excellent symmetry means that if you were to look at all the facets and compare to one another, they would look very uniform with very little variance.. This dosn't mean however the diamond is not cut too deep or too shallow.
The gradation for the symmetry of a diamond is Excellent, Very Good, Good, Average, Poor.
Diamond Polish
Polish is a measure of the surface finish. If a diamond is only " Fair" or " Average” it does not allow maximum transmission of light though the facets, due to it's " rough" surface texture. “Very Good” or “Excellent” Polish by most lab standards is usually the top cut grade which is mostly preferred. Polish graded above “ Good” is not detectible to the naked eye.
The gradation for the quality of the polish of a diamond is Excellent, Very Good, Good, Average, Poor.