This question came from a reader of my article about Diamond Cut.

Q: that’s great, but what about fancy cuts? I am interested in a rectangular radiant cut diamond, and the amount of conflicting information on what angles and proportions are preferable is very confusing.  Some sites say that to be a very good cut, the table and depth percentages need to be less than 65%, other say that for a rectangular radiant, depths of more than 70% are preferable.  Help!

A: You bring up a very good point. Aside from perhaps length to width ratios, there really aren’t any universally accepted ideal cut standards for any fancy shape.  There was a lot of talk at one point of some of the labs coming out with Princess and Emerald cut grading scales, but I’m not sure whatever came of it.

For the straight-sided brilliant cuts (like radiant and princess), the general rule of thumb as I recall is 70/70 (70% depth, 70% table).  But unlike the 60/60 rule with rounds, there is significantly more leeway. For example, a princess cut can range from about 65% depth to about 75% depth and still look great, but a 65% depth round stone will be just shy of hideous. With Radiants, the ideal range is probably more like 60% to 74% depth.

With the round-sided fancy shapes (like oval and marquise), you either really need to see the stone, or make sure the online vendor you’re buying from has a good return policy.