Out of the most respected labs in the world, no lab apart from the AGS gives princess cuts a cut grade. Therefore, for the most part, consumers usually are left in the dark about the true cut grade of princess cuts. Whilst some dealers may go merely by table and depth percentages, and others by crown height, these numbers alone aren’t the best indicators of a superior princess cut.
There are various ways to judge the cut quality of princess cuts, these include:
- Using the table and depth percentages. Not very accurate, but can be used to reject those stones with large tables that also have large depths (eg 80/80). Bear in mind there is no “ideal” total depth for princess cuts – I’ve seen ones with 77% depth return as much light as ones with a 70% depth.
- Using Sarin (proportion) information. Slightly more accurate, and can be used with DiamCalc to visualise the actual light return.
- Using raytracing software based on actual proportions. There are a few software packages available, the best ones being DiamCalc and The AGS Perfomance Grading Software.
- Using reflector technology such as the Ideal-Scope and AGS ASET. The AGS ASET is said to be better for grading fancy cuts as it has a three colour filter that give more details as to what angle light is leaving the diamond from.
Compared to round brilliants, nailing down ideal princess cut proportions is not that simple. Even cut enthusiasts would admit it is difficult to tell a bad stone from a good stone based on its proportions. Therefore, it is crucial when buying a princess cut to assess the stone’s overall performance through tools such as the Ideal-Scope, AGS ASET or computer assisted optical tools such as the BrillianceScope.
In terms of clarity, it is common to have inclusions in a princess cut are on the crown (top side) of the diamond. Therefore, if you look through the side of the diamond, you will probably be able to see SI-graded crystals and clouds with your naked eye. Furthermore, if the stone has a black inclusion in the centre, it will most likely not be 100% eye clean, as the inclusion will likely be directly under the table, as opposed to being deeper down the stone.
In conclusion, buying a good princess cut is a lot harder than buying a round brilliant. The light performance may be difficult to ascertain, making the comparison of stones between vendors difficult. This is especially true if one vendor has little information about the stone or gives a cut grade based on table and depth percentages without any other tools to assess the cut. When buying SI clarity stones, especially online – make sure they don’t have black (pique) inclusions in the center, as more often than not, they won’t be eye clean.
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