In general, even on a white, modern cut round it's not very useful as a rule of thumb. On a cut other than round and/or on a fancy colour stone, it's probably worse than not very useful, and it is positively misleading.
I'd much rather use - again, on a white modern cut round brilliant - a GIA or AGS cut grade. It takes into consideration many more factors, and as a consistent, synthetic indicator of a stone's proportion quality is as good as one can get today.
You don't want to hear it, but it does depend on what you are looking for. If what you want is a round diamond that "looks great", pick a stone that is graded on cut by GIA Excellent or Very Good, or an AGS 0-2. Colour and clarity optional, but probably clarity not lower than SI2 (or VS2 if you are buying totally sight unseen and don't want to risk having to return the stone). Then negotiate the best price you can. Don't go further than the lab cut grade; it will look more than good enough compared to a random pick not to let you down in any circumstances, and it simplifies the choosing task enormously. Here is one that fits these criteria, and it also happens to have a depth slightly smaller than its table. http://www.abazias.c...0809524&flag=dr
If what you want is the ne plus ultra
of diamond cut, then that's the point where a "simple" cut grade (and indeed any information on a lab report) just won't do. It is also the point at which your personal preferences play a part. My "best diamond in the world" is not Laurie's (this just to pick a person here that seems to have tastes in cut rather different from mine), and probably not yours. For some people, it is very possible that "the best" will involve a table larger than depth.
Edited by davidelevi, 03 April 2011 - 03:54 AM.