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Diamond Depth - Are we too strict?

gm89uk

Brilliant_Rock
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Often on PS you'll have people stating the set guidelines of depth up to 62.3 (including myself). If a particular diamond has a depth of 62.4 or 5 but is otherwise perfect, the advice generally would be "shame about the depth and loss of spread because it's going to face up slightly smaller than another ideal cut diamond with depth <62".

I've discredited multiple diamonds myself that looked great because I believed they were too deep. However, I've come across a few diamonds to challenge this.

Look at these two diamonds:
(1.5ct, 62.6% depth, 7.315mm spread)
https://www.jamesallen.com/loose-diamonds/round-cut/1.50-carat-f-color-si1-clarity-excellent-cut-sku-4616778

(1.5 ct, 61.8% depth, 7.315mm spread)
https://www.jamesallen.com/loose-diamonds/round-cut/1.50-carat-e-color-si2-clarity-excellent-cut-sku-4046910

They are the same carat weight, the same mm diameter, but one has a depth of 62.6 and the other 61.8.
Although increased depth generally means that spread is reduced, this is definitely not the rule.
Another few potentially more useful example is typical 60:60 diamond with good numbers:

Supposedly great spread:
(1.00carat, 60.8% depth, 6.445mm)
https://www.jamesallen.com/loose-diamonds/round-cut/1.00-carat-j-color-si2-clarity-excellent-cut-sku-3646773

(1.00carat, 60.9% depth, 6.43mm)
https://www.jamesallen.com/loose-diamonds/round-cut/1.00-carat-j-color-si2-clarity-excellent-cut-sku-4259635

(1.00, 61.3%, 6.42mm)
https://enchanteddiamonds.com/diamonds/view/Round/GIA-Certified-1-0-Carat-E-Color-VS1-Clarity-Diamond-6040967Z3


Compare it to

(1.00, 62.7%, 6.415mm)
https://www.jamesallen.com/loose-diamonds/round-cut/1.00-carat-g-color-vs2-clarity-true-hearts-cut-sku-2536114

(1.00carat, 62.1% depth, 6.455mm)
https://enchanteddiamonds.com/v/196140Z78

(1.00carat, 61.8% depth, 6.435mm)
https://www.hpdiamonds.com/en-us/diamonddetail/HPD8709

(1.007carat, 61.9% depth, 6.43mm)
https://www.whiteflash.com/loose-diamonds/round-cut-loose-diamond-3929342.htm

These are just few examples of how depth =/= spread, and the deeper diamonds really hold their own against the shallower depths. I couldn't believe the spread the 62.7% depth diamond had!

How can this be? Well it depends where the carat is going. The girdle being almost equivalent to a cylinder will take the most carat per % increase depth. This is generally followed by the crown taking up the second most carat per crown height % increase, then the pavilion taking the least for every extra pavilion height % you add. Generally, a small table will add little carat relative to the depth %, so although it may make the depth look bad, the spread isn’t affected much.
This means diamonds with larger tables may not have the amazing spread as their depth % may seem to indicate compared more traditional H&A diamonds with ~62% depth

So how to accurately measure spread? We should technically measure spread relative to carat, not depth % (although more of a nuisance to calculate):

Average Diameter(mm) / CubeRoot(Carat) = Average Diameter PER carat
We apply the cuberoot as carat is a measure of mass (3D), while diameter is just a length (1D).

If we say a typical ideal diamond would have these stats (on a GIA cert)

Table: 57
Crown: 34.5
Pavilion 40.7
LGF: 77
Star%: 50
Depth: 61.5%
Girdle: 3.5%
Would have 6.42mm spread.

We can then roughly say then any diamond around 6.42mm spread per carat has average spread (relative to the diamond above, or any diamond well below 6.40mm spread per carat could have better spread).

Example (comparing different carat sizes):

(1.77 carat, 61.8%, 7.82mm spread) = 7.82 / 1.77^(1/3) = 6.465mm per carat (excellent spread).
https://www.jamesallen.com/loose-diamonds/round-cut/1.77-carat-h-color-si1-clarity-true-hearts-cut-sku-3187056
vs
(1.72 carat, 61.9%, 7.645mm) = 6.38mm per carat (not as good).
https://www.jamesallen.com/loose-diamonds/round-cut/1.72-carat-h-color-si1-clarity-true-hearts-cut-sku-3187063

My take home message really is when the table is small, be a bit more forgiving of the depth, and you might not be missing out on any spread when the depth goes beyond the 62.3% magic number if the table is 54-55% vs 57-58%. I don’t intend this to influence your daily searches, but I certainly found it interesting.

I first thought of the idea when I saw @Rhino AVR post (https://www.goodoldgold.com/the-august-vintage-european-cut) which includes “Key thing I'd like to point out here. The August Vintage European Cut, while having a 64.4% depth has an average diameter of 8.15mm. The GIA XXX's in the above query have 62.x% depths with an average diameter 8.10mm. :) How can that be your ask? You can call that a small miracle.” I thought the same must apply to a lesser extent to MRBs and I found it does!
 

flyingpig

Ideal_Rock
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Nov 7, 2015
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2,026
Interesting topic. I always suggest depth less than 62.3%. But personally, I set it to less than 62.5%.
I understand what you mean. A diamond with depth larger than 62.3% can still have a decent spread. As you know, depth% is depth/diameter*100. The result is more affected by the numerator than by the denominator. So, a high depth % does not necessarily translate to a significant loss in diameter.

Than, why suggest depth less than 62.3%?
Set the depth range to 62.5~62.8%. Steep/Deep stones dominate (or slightly thick girdle)
Set the depth range to 60.0~60.5%. Shallow crowns dominate.
Of course, there are always some exceptions.

We cannot set crown/pavilion ranges for our search. But adjusting the depth (along with table %) kinda achieves a similar thing.
 
Last edited:

diamondseeker2006

Super_Ideal_Rock
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Jan 11, 2006
Messages
55,105
Yeah, I usually set my max depth at 62.4 and table at 58 when searching for other people. I don't consider 1 ct round brilliants under 6.4mm, though, for modern rounds. But you are right that there are stones that are exceptions.

AVR's do face up smaller in general than hearts and arrows rounds. But they are a totally different cut with higher crowns and greater depth. Both of mine have 64.x% depths and about 19% crowns and they face up well, but still about .1mm less than a H&A round.
 

cflutist

Ideal_Rock
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Jul 12, 2004
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3,821
For mind clean reasons, I personally won't go above 61.8% depth on a CBI diamond.
My 2.793 F-VS1 is 61.6%
My 2.314 Q-VVS2 is 61.0% (nice spread)
My 2.210 F-SI1 is 61.6%
My 0.532 F-VS1 is 61.3%
My 0.530 F-VS2 is 61.4%

I am sure that 62% WF ACAs and CBIs have great optical performance. I just can't buy one myself for psychological reasons, my bad. :read: So no, I don't think we are too strict.
 

bmfang

Brilliant_Rock
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Max depth for me is usually 62.5 though at times I have left it up to 63 just to see what is out there.
 

Lorelei

Super_Ideal_Rock
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Yes we can be too strict on depth, absolutely! But it depends on the buyer, not everyone wants a superideal and for SI round specs, you don't normally see those go below 62 if ever but for those with less exacting standards, if everything else about the stone works well, then a little leeway can be used certainly. For fancy shapes and the new superideal fancies, it's a whole 'nother ball of wax.
 

flyingpig

Ideal_Rock
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The other ideal ranges such as CA 34-35, PA 40.6-40.9, TB 54-58 can be too strict as well.
We know 35.5~36.5 or 33.0~33.5 CA can be good. TB 60 can be good. PA 40.5 or 41 (or even 41.2) can be good.
Some people criticize PS and its member for giving out cookie cutter advice all the time.
But you gotta draw the line somewhere. And it is sensible and responsible to give noobs the safest advice.
 

Lorelei

Super_Ideal_Rock
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The other ideal ranges such as CA 34-35, PA 40.6-40.9, TB 54-58 can be too strict as well.
We know 35.5~36.5 or 33.0~33.5 CA can be good. TB 60 can be good. PA 40.5 or 41 (or even 41.2) can be good.
Some people criticize PS and its member for giving out cookie cutter advice all the time.
But you gotta draw the line somewhere. And it is sensible and responsible to give noobs the safest advice.
Absolutely, not everyone wants the top notch cut, there has to be room for all and it's a shame if noses are upturned at a perfectly beautiful stone but the proportions aren't quite ' Pricescope Perfect.' So it's good to find out what cut standards the buyer actually wants or risk throwing out good stones. But the above proportions, which have been around and served many for donkeys years now, do at least get you on the putting green and sometimes get you a hole in one - if you like golf.
 

Rhino

Ideal_Rock
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Excellent observation gmuk. As noted it isn't *always* the case but it is in others as you have pointed out.
 

Lorelei

Super_Ideal_Rock
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Don't forget the other direction....based on my training, anything over 61% seems deep.
Absolutely David and especially it's good to hear from those like yourself in the trade, invaluable in fact!
 

Wink

Ideal_Rock
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Fred Quellar, wrote a book, "How to Buy a Diamond" many years ago which was really a treatise on "Why I am better than all the rest and everyone else is a crook". He also posted Rap sheets in the book, so that no matter what another jeweler would try to sell the diamond for, the outdated Rap sheets would reinforce that the other Jewelers were trying to rip off the buyer.

Also famous for insisting that almost any diamond you asked him about had a warped girdle.

Now that you know who Fred is, I think you should go on the war path and go after Jonathon. That way we can have some good old fashioned entertainment since you and Karl are no longer providing it for us.

Wink
 

Karl_K

Ideal_Rock
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LOL Dave you sound like Fred "The Diamond Guy". :eek2::lol-2:
That is just plain rude.
Being trained to like 60/60 diamonds by the premier diamond company of that time period is is a very very far cry from Fred.
 

Karl_K

Ideal_Rock
Trade
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On the subject of depth, yes depth does not always equal spread even in rounds.
It is certainly true in fancies.
I have been preaching it for years.
 

Maggiemeans

Shiny_Rock
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Okay as I reveal more of myself on PS I am going to admit something... I READ THAT BOOK! Guessing around the year 2000ish? From a consumer's perspective it put the fear of God in you that everyone other than Fred was evil and trying to take advantage.. subliminally directing business to him. It explained the basics.. internet was not so strong at that point. So it gave basics. I don't think it got into Crown and pavilion angles and such.. unfortunately I threw it out years ago but for pure entertainment I wished I had kept it. I think his company was called Diamonds International.
 

Rockdiamond

Ideal_Rock
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Ironic- but this Fred guy really sounds like posts sometimes seen on PS where consumers are warned against aspects of cut that have been extrapolated- and possibly incorrectly. I think this is part of gm89uk's point
 

gm89uk

Brilliant_Rock
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As you know, depth% is depth/diameter*100. The result is more affected by the numerator than by the denominator. So, a high depth % does not necessarily translate to a significant loss in diameter.
Yes absolutely, in fact a higher depth % can have better spread than a lower depth %. But unfortunately you are right, the number of bad eggs as the depth% goes up, increases dramatically.

@cflutist, nice collection! In all honesty CBIs >62 depth are a rarity and I've only ever come across a couple.

Essentially, I know many PSers find a good stone, and just wish it was on the right side of 62, but if it's a little deep, that's not always the whole story. If the other specs are right, you may not have compromised spread. It's just a post to give some mind-cleanness to fellow PSers..

AVR's do face up smaller in general than hearts and arrows rounds. But they are a totally different cut with higher crowns and greater depth. Both of mine have 64.x% depths and about 19% crowns and they face up well, but still about .1mm less than a H&A round.
Which is an amazing accomplishment to only come out 0.1mm smaller, with a 64.x% you'd expect much more damage to the spread (if they were MRBs with 64.x%). AVRs with 64.x% have higher crowns and smaller tables, and higher pavilions, come out with the same spread as a 62.x% MRB. I find that statistic somewhat boggling!
 

pfunk

Brilliant_Rock
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Yes, I think we can be too strict on depth at times. My wife’s diamond is an example of something that wouldn’t be paid too much attention to here, but nonetheless would have the same effect as a diamond that is too deep.... a slightly thick-slightly thick girdle. It robs a bit of weight too, but it isnt called out as much and is not something that is simple to screen for when searching. Unless you get too out of hand, a little extra depth isnt going to lead to a reduction in mm spread that you are going to be able to see. Might you pay a little more for weight that isn’t translating into spread?... sure. But you wouldnt have seen the extra spread in most cases (i.e. 62.4% vs 61.5%) and if the other aspects of the cut are top notch, then it still might be the better stone. I still think it is wise to set an overly strict guideline when recommending search parameter for a novice, whereas if you are going to make personal recommendations for them and you know what you have to be careful of when you start combining the depth with other aspects, then it is ok to widen the search so you don’t miss those winning stones out on the fringes.
 

Lorelei

Super_Ideal_Rock
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This is interesting actually because it seems people have relaxed somewhat over the years where depth is concerned, this is a good thing. I can remember when going much below 61 in a MRB was a big no no, especially in Superideals, which is fair enough with those. But for non superideals that are still cut for optical performance in mind, I think a little leeway is a good thing and of course for fiery ideals, the extra depth often goes with the territory.
 
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