How can you tell if a diamond is too deep?

[PrincessBride]

Hi! How can you tell if a diamond is too deep? What is considered too deep? My bf and I are looking at diamonds and we understand the 4C as well as symmetry, polish and girdle. I''d like to understand more about the depth and table. Anyone care to explain?

 

[valeria101]

Just my 0.2 as usual...

"too deep" may show either in the size or brilliance of the stone or boths:

#1. Extra depth eats up material so the diameter of the diamond turns out smaller than it might for the given weight.

#2. Extra deep pavilion gets too steep to allow light to reflect back to you from the center of the stone - so you get a dull patch under the table.

These two however only explain why very, very deep princess cuts (about 80% deep) get discounted for their depth. In the more practical, middle range (60%-75%, say - after the AGA charts) less deep is not always better... and this makes things more complicated than table and depth can explain on their own.

Speaking of depth: total depth adds up three things: crown height, girdle thickness and pavilion depth. It may be that the sum (total depth) is better to be less, but this is not true for each of the components.

Higher crown does add to depth, but also helps allot with brilliance and fire which is good.

The girdle becomes fragile is extra thin, and for a stone with sharp corners (princess) thin may be less durable too. Thick and very thick girdle is already a bit much, I thing, and a thich girdle actually eats up more material than a bit of extra depth - so if size for weight is a concern thick girdles are out (IMO) first and the last couple of depth % second.

Pavilion depth is too much when it doesn't match the crown height well - these need to keep up a certain relationship to produce desirable optics.

The AGA standards ballance all these as much as the typical Sarin data allow. The table & depth and girdle numbers on a GIA cert say even less - there is no hint of crown and pavilion depth and this means there is no knowing if the total depth keeps up with the right crown & pavilion relation. To really pin down the looks, allot more detail is needed, and neither GIA stats, or AGA extended version describe all that goes in a princess cut.

A complete report looks like this: see the DiaVision file.

That many numbers are hard to interpret off the bat, but one can literally recostruct the diamond and analyze it's optics based on such a detailed report. There is software for this. Not that's not complicated - perhaps a bit too much ?

It's way easier to look at the stone than call in all the relevant numbers, and this is done by direct cut grading tools - you know, the 'scopes (Ideal Scope, Brilliance Scope and... who knows, there must be other out there too). The normal diamond photos depend too much on lighting, camera and what not, so they are not very informative. Same goes for looking at diamonds in person - everything looks good under strong lighting (popular in jewelry stores, btw) and with just a couple of stones at hand one may never see what they are missing. These 'scopes standardize diamond imaging - rather useful, I think.

So... well, there are a few ways to check if depth is too much. With GIA stats only, you can tell if it makes a certain stone smaller than it should. Just look at total depth and girdle thickness. If depth affects brilliance, there is no way to know lookng at the lab report only.

Hope some of this helps

 

[oldminer]

Ana:

I couldn''t have said it better. You are one of the few who have "gotten it right".