Scientific evidence that better cut = bigger size!

[someguy]

So there you have it folks! This is scientific proof that better cut=bigger looking diamonds!! For those of you who are wondering where this comes from, I went to Blue Nile (since they have a vast selection of diamonds) and looked at the size parameters for 25 random diamonds from each cut group: fair, good, very good and ideal.

Here are my observations:

1) As cut quality increases, so does size of the diamond!! You’ll notice that this trend starts to level off once we get to the ideal cut range. I suspect that this would be even more apparent if I included data for the signature ideal diamonds.

2) As the cut quality increases, the error bars get smaller. This means that there is less and less variation in the size of diamond as you increase the cut quality. Statistically, roughly 2/3 of diamonds for a particular cut quality should fall within the range established by the error bars. However, I’ve seen some really extreme cases! One of the poor cut 1ct diamonds was actually something like 5.88*5.83mm!!!

This goes to show that, even if you believe that size is everything, you should really consider getting one of the better-cut diamonds. This also suggests that more emphasis should be placed on the actual physical dimensions of the diamond instead of the carat weight. When I have more time, I’ll try to do something similar for diameter vs carat for the different cut classes. That should be interesting because I’ve noticed that the super ideal cut diamonds that are advertised by the pricescope vendors in the 1ct range usually have a diameter around 6.5-6.6 mm. I’ve also seen many 0.9ct diamonds that are 6.3 mm in size, which compares very well to the lower cut 1ct stones that are being sold everywhere. I hope that everyone finds this interesting!

 

[someguy]

Sorry about the formatting guys, I'm just not very good at pasting things from word yet...

 

[FOODIE]

May I be the first to chime in ... this IS interesting. My guess is that optimum cut should represent the ability for light to bounce around the inside of the stone.

When you say the better cut represents a bigger diamond, do you mean a bigger table?

 

[strmrdr]

While you raise a good point its not quite accurate.
A too shallow diamond will be larger in diameter than a properly cut diamond.
Or one with too much material removed from the pavilion will also.
It just happens that a lot of non-ideals are fat paviloned or deep or both that makes them heavier for any given diameter.
Its the balance of the cut that makes a great cut.
This is why some diamonds that arent in the ideal range by the numbers can be awesome performers. They just happened to hit a great balance point that is outside the accepted ideal range.
They usualy happen by chance where ideal range and in particular super-ideal range diamonds have the extra time and care in cutting to make them that way.

 

[pqcollectibles]

Mara's diamond is a prime example of an exception to you thesis. Weighs 1.2 carat but has the diameter of a 1.35 carat. Mara's diamond is a very good performer as well. There are many similar exceptions, and each diamond needs to be evaluated on a case by case basis. Some will be great like Mara's and some will be duds.

 

[Jim-Schultz]

Someguy,

Your analysis is pretty much correct. It's much more common for a diamond to be considered a poor cut because it's too heavy, rather than being too spread. This drags down the diameter for the poor diamonds. Most people don't realize it, but depth percentage is just a simple calculation - take the depth of the diamond in millimeters and divide it by the width of the diamond in millimeters. It's a little different for the fancy shapes, but the idea is still the same.

In order to make sure you get as much round diamond as you pay for then, we usually advise folks to try and find round diamonds with depths from 60-62%. There's nothing magic about that range, although it's surprising how many diamonds with those depths and smaller tables score fantastic on the HCA.

 

[someguy]

Hi, I just reread my post, and I wanted to make myself a little more clear on what I was trying to say. I kept on hearing that a well cut diamond will appear to be larger than most diamonds of the same weight that are out there. So I thought that I'd test it out and see if this was true. The conclusion that I draw from this data is that this is indeed true. This graph allows us to compare a particular diamond to what's out there, on average. I'm sure that there are a lot of exceptions, but the graph just gives us an idea of what's out there. When I think of purchasing a 0.9ct diamond that 6.3mm in diameter, it's size really doesn't look as bad anymore

!

 

[in_need_of_help]

This is scientific proof that better cut=bigger looking diamonds!!


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I would not go so far as to use your findings as "scientific proof." The trend is an interesting observation.

 

[valeria101]

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I would not go so far as to use your findings as 'scientific proof.' The trend is an interesting observation.

----------------[/quote]

??? doesn't "trend" refer to a time variation of something ???

Well, this could obviously be some empiric proof for STRMDR's theoretical hypothesis that 'bad' cuts could either be too deep or too shallow compared to the current "ideal cut range" that PS recognizes. The chart simply shows that the premier motvation to cut diamonds with lesser light return is to keep that weight on. And this is something I keep hearing here too... I would further hypothetize that the current pricing 'per carat' of diamonds produces this bias. If, say, diamonds would be per unit of diameter length, we'd probably see more flat than deep "bad" cuts.

These comments leave aside the constraints that the shape of the rough may pose: but I suspect that oly a few of diamond rough pieces come in anything close to their theoretical crystal shapes... so I have no idea wether the shape of rough gives further incentives to cutting factories to make diamonds deep rather than sparkly. After all, I could only imagine what a mayhem would result from pricing cut diamonds for something not related to the pricing of the rough (light return as oposed to the speculative return, color&clarity range and weight).

Maybe some industry insider would care to make light here...

By the way: Are those confidence or variance intervals (on chart)?

 

[bof]

i would hardly call this scientific evidence. if you wanted to test your hypothesis that better cut diamond appear larger you need to have a sample group "look" at different sized diamonds of different cut qualities and quantify their impression of size.

conversely if your argument is that better cuts have larger diameters, previous posters have already pointed out that a badly cut diamond with a shallow depth will have a much larger diameter and thus appear larger.

you do however make some interesting observations...

bof

 

[valeria101]

Oups, no I got to read the observation No2) about the 'error bars': of course they get smaller as the quality increases! As far as I know these 'good', 'ver good' and 'premium' categories are DEFINED by cut parameters which include depth. This is not true for the 'Fair' class, which includes diamonds basically out of the range of the higher categories. This holds even if the quality categories are based on crown & pavilion angles (the depth feasible with these restricted angles can very only within a certain range too).

Why are not more shallow RBC diamonds out there (or in the PS listing)? This I would like to know...

 

[someguy]

Sheesh, tough croud

! I'll admit that I was maybe a little too sensationalistic... Like Valeria and Jim pointed out, this doesn't mean that a poor cut will certainly look smaller (here I equate smaller diameter to smaller apparent size). What it does show is that most poorly cut diamonds are too deep. If you accept that most diamonds out there are poorly cut, then you can only come to the conclusion that, on average, these will appear smaller than a well cut rock when viewed from above.

I found these results interesting because I now can believe it when people tell me: "Yeah, but that 0.9ct super ideal cut diamond will look just as big as most of the other rocks out there."

Valeria, it seems that I messed up a little with the error bars. I thought that I put the standard deviation, but instead I used the standard error (which is the standard deviation / sqrt(N), where N is the sample size and is in this case 25)... This is what happens when I try to do something fast

 

[DiamondExpert]

Please don't take these comments personally, but this is my knee-jerk reaction to your "scientific" study..

It seems that since the cut quality terms you use are not defined in any way they may be misleading...folks will be looking for 1ct. stones with 6.40mm diameters as "ideal" - I hope not.

And, what is meant by "ideal" - surely this is not the AGS-defined term - a quick survey of 9, 1ct., AGS0 H&A stones reveals a value of 6.52+/-0.02SEM!

According to your "data", if "ideal" is around 6.4mm (and it certainly is not as AGS defines it) and 6.5-6.6mm is "super-ideal"(as you suggest), then folks will be looking for 6.7-6.8mm "super-duper-ideal" cuts!?...I hope not!

I am not having a "eureka!" feeling here...if your point is that better cut rounds are not cut as deeply as poorly cut ones, then you have demonstrated the obvious, albiet in an indirect manner...but that's OK if it helps someone think about these things..