Angles of the Cut

The first measurement to take is the degree of the angle from the table to the girdle of the stone. This is known as the top critical angle and should be 34.5 degrees. Underneath the girdle, the bottom angle from the girdle to the point of the stone is also a critical angle and should be cut at 40.75 degrees. A further measurement is that the girdle should be about 1% as thick as the diameter of the stone, although this is not quite as critical as the other measurements and can be judged by the eye after a bit of practice.

A stone which is not cut with the critical angles in the right degree, will either be shallow cut or deep cut and will not reflect the light back through the center of the stone (the table of the stone) with the same brilliance as a stone that is cut to the correct angles.

If the stone is shallow cut, the light will reflect off the edges of the stone but not through the middle. If it is cut too deeply, the center of the stone will appear to be dark and it is called "heavy." In the past some cutters cut the upper angles at a less than 30 degree cut. This spread cut helps hide deficiencies in a stone but makes the girdle angles sharp and likely to be broken or chipped and the stone is not as valuable as a normally cut stone.

If the correct tool for sizing angles is not available, one can estimate that if the table appears to be larger than it should, and the width to height (that is the depth spread ratio) is below 60%, one can assume that the critical crown angles are shallow.

It is possible to polish a diamond to a high degree to compensate for shallow or deep cut angles at first glance and make the stone appear to be more brilliant than it, in fact, is. If the stone is chosen for investment quality, a measurement of these angles is almost essential.