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Evaluating Spread

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ILikeBond

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I often hear things like, "I would pass on that stone, at X carats, the spread should be about X.X, but this is only Y.Y." I understand the concept that ideal proportions will result in an optimum spread (and in particular that crown angle [low] + table % [high] determines spread). What I''m wondering is if there is a standard formula or table or other resource for being able to tell how close a given spread is to "optimum."

This seems easy enough at standard intervals - I think 1 carat should be about 6.4 mm and 2 carats about 8.2 (
).

But how can I compare apples to oranges when working outside standard intervals? For instance, how can I tell if a 1.37 carat diamond measuring 7.12 x 7.07 has a better spread than a 1.93 carat diamond measuring 7.97 x 7.92? (totally hypothetical situation here - I''m not interested in which of these diamonds actually has a better spread - just in the process to determine it)

(I think its redundant to say that since this is "spread," I mean to suggest how do I tell which has the better spread relative to the stone or the carat weight - obviously the second one is bigger overall).

I''ve looked through the pricescope tutorial, etc., but have not found anything.

Thanks!
 

JulieN

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If you Pricescope your diamond, a spread...differential(?)/factor comes up, the standard being a Tolkowsky. There is another formula that works quite well, can''t remember it right now.
 

ILikeBond

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Date: 11/27/2006 6:05:10 PM
Author: JulieN
If you Pricescope your diamond, a spread...differential(?)/factor comes up, the standard being a Tolkowsky. There is another formula that works quite well, can't remember it right now.
Do you mean using the HCA, the difference between an "excellent" and a "very good"? I was hoping for something more specific than that.

Can anyone else comment or point me toward a resource? Thanks!
 

ILikeBond

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Thanks, Julie - but what if a diamond is not listed on this, nor one that has its same carat weight and dimensions?

Thanks for the help.
 

JulieN

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in that case, a combination of mental sort, averages, and knowing that smaller table results in more depth for given angles should help.

you do this for a while, you get a feel for it.
 

oldminer

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Spread is the depth percentage. You calculate the depth percentage based on average girdle diameter of a round or the width to depth ratio of a fancy and you get what amounts to a percentage which will let you compare "spread" in a simplistic manner. That's all there is to it. You need the length, width and depth measures to do this. Its simple division.
 

Paul-Antwerp

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Date: 11/28/2006 9:22:22 AM
Author: oldminer
Spread is the depth percentage. You calculate the depth percentage based on average girdle diameter of a round or the width to depth ratio of a fancy and you get what amounts to a percentage which will let you compare ''spread'' in a simplistic manner. That''s all there is to it. You need the length, width and depth measures to do this. Its simple division.
ABSOLUTELY NOT. At least not in a fancy shape.

In a princess-cut, there is no direct relationship between depth percentage and spread. The same is true for other fancy-shapes.

What Dave says, amounts to saying that the spread of a princess (diameter 5.40x5.40) with a depth of 76% is less than the spread of a princess (same diameter 5.40x5.40) with a depth of 66%. This is UTTER NONSENSE.

Live long,
 

ILikeBond

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Date: 11/28/2006 9:22:22 AM
Author: oldminer
Spread is the depth percentage. You calculate the depth percentage based on average girdle diameter of a round or the width to depth ratio of a fancy and you get what amounts to a percentage which will let you compare ''spread'' in a simplistic manner. That''s all there is to it. You need the length, width and depth measures to do this. Its simple division.
Ah! - Makes sense
(for a round
- that''s what I was asking about. I won''t get in the middle of the fancies debate, which I know nothing about
). Its funny how we can overcomplicate things sometimes.

Like now. Just to confuse things: So is there a theoretical "optimal" spread at a given carat weight? If so, my guess is it would be based on ideal depth percentage, so maybe there''s actually a range of "optimal" spreads for a diamond at a given carat weight somewhere between about 59-61.8% depth or so?

You identify 3 variables - (1) depth %; (2) average girdle diamater; and (3) carat weight.

If you know all 3, you can calculate a ratio and compare it to another diamond of different carat weight; but what if you only know (1) and (3) and want to determine what (2) should be?

Thanks!
 

JulieN

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The variables are crown angle, pavilion angle, table, girdle, and culet.

If you know 1 and 3 you cannot come up with 2.

Your optimal spread depends on preferences. You cannot expect a FIC with 52% table and 63% depth to have the same spread as BIC with 58% table and 60% depth.
 

Regular Guy

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I don''t have the answer, but the question makes me want to hum a few bars....

a) interpolate (found in the FAQs)

and then, for comparing absolute differences...

b) Garry''s math
 

ILikeBond

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Date: 11/28/2006 5:09:35 PM
Author: JulieN
The variables are crown angle, pavilion angle, table, girdle, and culet.

If you know 1 and 3 you cannot come up with 2.

Your optimal spread depends on preferences. You cannot expect a FIC with 52% table and 63% depth to have the same spread as BIC with 58% table and 60% depth.
Then why do people say, "at X carats, that spread should be closer to y.y millimeters, but that diamond is only z.z millimeters."? I understand that because there is no single ideal set of proportions, but rather a range of ideal proportions, that it cannot be said a single spread would be optimal, but there should be a range that falls within optimal consistent with the range of optimal round diamond proportions - probably TIC rather than BIC or FIC.

So, for instance, I''m sure its possible to say that "a 1-carat stone [variable 3] with proportions within the TIC ideal range [variable 2] should have a spread of between 6.4 and 6.6 millimeters."

I guess that''s what I''m looking for. I understand that by limiting variable 2 to depth - rather than all the key proportions you identify - I''m oversimplifying, but I think its fair to just call the variable "TIC ideal proportions."

Put another way, if I have a 1 carat diamond with an 8 mm spread, I know its wayyyyyyy shallow, and if I it has a 3 mm spread, I know its wayyyyy deep (I know - extreme, hypothetical example).

Its funny, when I started this thread, I thought someone would be able to post a link to something where I could just plug in carat weight and on the basis of well-known TIC ideal proportions, it could spit out for me an optimum spread range. That way, if I was looking at 2 different diamonds, I could tell (1) if both fall within optimum ranges; and (2) if they both do, which one nevertheless appears bigger because it is at the higher end of the optimum range.

Am I being completely non-sensical or making some sort of faulty assumption or something? I''m not very math inclined, so that could very well be the case...
 

ILikeBond

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Date: 11/28/2006 5:48:30 PM
Author: Regular Guy
I don''t have the answer, but the question makes me want to hum a few bars....

a) interpolate (found in the FAQs)

and then, for comparing absolute differences...

b) Garry''s math
Thanks! Those threads were helpful.


(in particular, one thing that didn''t occur to me, which Gary mentions, is that the difference you see isn''t just the difference in mm, but the difference in surface area, so its actually the mm squared)
 

JulieN

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It is frustrating when people say it should measure something else, when the proportions are perfectly fine. I do say it myself, usually if someone is trying to hit a certain carat weight. But when somone refers to X diameter, it is usually Tolk.

BTW, the three simplified variables for spread are table, girdle, and depth, then. Carat weight is not a variable for spread, though you may consider it a variable for volume-->size
 

ILikeBond

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Date: 11/28/2006 6:11:24 PM
Author: JulieN
It is frustrating when people say it should measure something else, when the proportions are perfectly fine. I do say it myself, usually if someone is trying to hit a certain carat weight. But when somone refers to X diameter, it is usually Tolk.

BTW, the three simplified variables for spread are table, girdle, and depth, then. Carat weight is not a variable for spread.
Thanks, Julie.

By way of background, I''m looking at a couple diamonds, all of which are ideal cut with proportions within the TIC range. Now, one is 2.09 with an 8.12 mm average spread. I''ve looked at several other 2.09s just for comparison and almost all of them are closer to 8.20+ average spread. And then to make matters worse, I''ll look and find something like a 2.08 with a spread of 8.30, or a 2.03 with a spread of 8.14.

Since diamonds are sold by price per carat, I''m just really wary - even though its a 6-point difference, why buy a 2.09 carat diamond that''s only spreading 8.12 if a 2.03 spreads higher, even if it is just minimal. At the prices per carat for 2+ carats, it could be a several-hundred dollar difference. I''d really just like to get the most bang for the buck. Remember - all these diamonds are "ideal" range. I also realize the perceived difference would be minimal b/c we''re talking about such small differences here, but if I''m purely looking at value, I''d like to know whether a 2.08 with a 8.3 spread is a better value than, say, a 1.94 with a 8.01 spread.

Also, I don''t get the second statement. How can carat weight not be variable? You wouldn''t expect a 1 carat diamond to have anywhere near the spread of a 2 carat diamond unless one or the other were REALLY ugly.. right? I realize, of course, that carat weight does not DETERMINE spread, as table and crown angle do, but carat weight certainly has a mathematical relationship to spread... I think.

Thanks for bearing with me, Julie. Hope I''m not frustrating you. I''m a noob, so for me its just part of the learning process and desire to get the most bang for the buck, even if it amounts to splitting hairs at this level.
 

Garry H (Cut Nut)

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Date: 11/28/2006 9:22:22 AM
Author: oldminer
Spread is the depth percentage. You calculate the depth percentage based on average girdle diameter of a round or the width to depth ratio of a fancy and you get what amounts to a percentage which will let you compare ''spread'' in a simplistic manner. That''s all there is to it. You need the length, width and depth measures to do this. Its simple division.
Hope you dont think I am picking on you Dave, but I dont think it can ever be that simple - Girdle thickness is the main place where weight is (or isnt).

Here is an example of a 60% table 60% depth % - it should be about 1.00ct at 6.47mm.

But it weighs 20% more because of the extreme girlde thickness.

I know this is an exaggeration - but it shows that depth % is not a real good predictor - even in rounds.

The lower right of the image shows the SPREAD factor on DiamCalc - it rates any diamond of any shape and cut to Tolkowsky. we think this is the best system for comparison - because it tells you a marquise might have 10% better spread, and an asscher 20% less spread etc.

sergey is also working on edge or border contrast and other issues where the percieved size can be effected by localized light return and contrast etc

spread 60 60 1ct.JPG
 

JulieN

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Date: 11/28/2006 6:26:11 PM
Author: ILikeBond

Thanks, Julie.

By way of background, I'm looking at a couple diamonds, all of which are ideal cut with proportions within the TIC range. Now, one is 2.09 with an 8.12 mm average spread. I've looked at several other 2.09s just for comparison and almost all of them are closer to 8.20+ average spread. And then to make matters worse, I'll look and find something like a 2.08 with a spread of 8.30, or a 2.03 with a spread of 8.14.

Since diamonds are sold by price per carat, I'm just really wary - even though its a 6-point difference, why buy a 2.09 carat diamond that's only spreading 8.12 if a 2.03 spreads higher, even if it is just minimal. At the prices per carat for 2+ carats, it could be a several-hundred dollar difference. I'd really just like to get the most bang for the buck. Remember - all these diamonds are 'ideal' range. I also realize the perceived difference would be minimal b/c we're talking about such small differences here, but if I'm purely looking at value, I'd like to know whether a 2.08 with a 8.3 spread is a better value than, say, a 1.94 with a 8.01 spread.

Also, I don't get the second statement. How can carat weight not be variable? You wouldn't expect a 1 carat diamond to have anywhere near the spread of a 2 carat diamond unless one or the other were REALLY ugly.. right? I realize, of course, that carat weight does not DETERMINE spread, as table and crown angle do, but carat weight certainly has a mathematical relationship to spread... I think.

Thanks for bearing with me, Julie. Hope I'm not frustrating you. I'm a noob, so for me its just part of the learning process and desire to get the most bang for the buck, even if it amounts to splitting hairs at this level.
2.08 with an 8.3 /diameter,/ not spread.

I use spread like Garry does in the above post; it is a factor, a calculated number, not a measured value. Carat weight has a direct relationship to volume. Determine the proportions that you want, then you can calculate the diameter. Then you can calculate spread. I say that carat weight is not a variable of the spread factor (in %) because you are comparing the diameter of two stones of equal carat weight.
 

JohnQuixote

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Date: 11/28/2006 10:12:00 AM
Author: Paul-Antwerp

In a princess-cut, there is no direct relationship between depth percentage and spread. The same is true for other fancy-shapes.
Paul was the one who brought this to the forum''s attention in 2005 ("it''s the end of the world as we know it").
Take for example the 3 princess cuts below.

Diameter is fixed at 6 x 6 mm. See how the spread can remain the same while the depth and carat weight change:
Example 1: 6.00 x 6.00 x 4.50 (depth 75.1%) weight 1.32 ct
Example 2: 6.00 x 6.00 x 3.92 (depth 65.3%) weight 1.16 ct
Example 3: 6.00 x 6.00 x 4.50 (depth 75.1%) weight 1.16 ct

Example 1





Example 2. Same spread, but completely different depth.



Example 3. Depth of 1 with ct weight of 2 (this is an unrealistic example but it demonstrates the point)


See this thread and the linked article for more.
 

ILikeBond

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Date: 11/28/2006 8:23:49 PM
Author: JulieN




Date: 11/28/2006 6:26:11 PM

Author: ILikeBond


Thanks, Julie.


By way of background, I''m looking at a couple diamonds, all of which are ideal cut with proportions within the TIC range. Now, one is 2.09 with an 8.12 mm average spread. I''ve looked at several other 2.09s just for comparison and almost all of them are closer to 8.20+ average spread. And then to make matters worse, I''ll look and find something like a 2.08 with a spread of 8.30, or a 2.03 with a spread of 8.14.


Since diamonds are sold by price per carat, I''m just really wary - even though its a 6-point difference, why buy a 2.09 carat diamond that''s only spreading 8.12 if a 2.03 spreads higher, even if it is just minimal. At the prices per carat for 2+ carats, it could be a several-hundred dollar difference. I''d really just like to get the most bang for the buck. Remember - all these diamonds are ''ideal'' range. I also realize the perceived difference would be minimal b/c we''re talking about such small differences here, but if I''m purely looking at value, I''d like to know whether a 2.08 with a 8.3 spread is a better value than, say, a 1.94 with a 8.01 spread.


Also, I don''t get the second statement. How can carat weight not be variable? You wouldn''t expect a 1 carat diamond to have anywhere near the spread of a 2 carat diamond unless one or the other were REALLY ugly.. right? I realize, of course, that carat weight does not DETERMINE spread, as table and crown angle do, but carat weight certainly has a mathematical relationship to spread... I think.


Thanks for bearing with me, Julie. Hope I''m not frustrating you. I''m a noob, so for me its just part of the learning process and desire to get the most bang for the buck, even if it amounts to splitting hairs at this level.
2.08 with an 8.3 /diameter,/ not spread.


I use spread like Garry does in the above post; it is a factor, a calculated number, not a measured value. Carat weight has a direct relationship to volume. Determine the proportions that you want, then you can calculate the diameter. Then you can calculate spread. I say that carat weight is not a variable of the spread factor (in %) because you are comparing the diameter of two stones of equal carat weight.

Ahh! So I was making a simple but crucial mistake, equating spread with diameter, when one is a proportion and the other is a measurement.


So I guess my question really is different - how do you determine the optimal diameter of a stone of a given carat weight.

This has been very helpful and interesting, though. Thank you Julie, and everyone else for the info and links - lots of good stuff to read!
 

oldminer

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A 1.00ct princess cut with a 66% depth will look:

1. larger than a 1.00ct princess cut with a 76% depth.

The exception would be a 66% depth stone with a bulging pavilion cut and a very thick girdle which keeps a lot of weight in the stone but does NOT show up in increased depth percentage. It is unusual, but could occur.

So Paul is technically correct. Its why he believes I am dead wrong with depth percentage screening and I will eventually switch to face-up surface area to weight ratio when I am able to. Meanwhile, depth percentage needs to be used as a screening tool for spread comparison. It isn''t perfect, but it works a good deal of the time.
 

oldminer

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Getting to the heart of the issue, the "optimal diameter", is defined by me as "the best range of spread for a given weight".

If you allow depth percentage to assist you, then the best range of spread is probably within the 2B range of the AGA Cut Class charts. Anything from 1A to 2B has a good opportunity for beauty and many people prefer spready stones to overly deep, non-spready ones. No wonder!

You can calculate spread with various geometric formulas on some shapes such as round, square and rectangular. It becomes a subject for direct measurement with marquise, oval, pear cuhion, etc. Spread based on depth percent gives a general clue, but is not a perfect tool.
 

Paul-Antwerp

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Date: 11/30/2006 8:58:01 AM
Author: oldminer
A 1.00ct princess cut with a 66% depth will look:

1. larger than a 1.00ct princess cut with a 76% depth.

The exception would be a 66% depth stone with a bulging pavilion cut and a very thick girdle which keeps a lot of weight in the stone but does NOT show up in increased depth percentage. It is unusual, but could occur.

So Paul is technically correct. Its why he believes I am dead wrong with depth percentage screening and I will eventually switch to face-up surface area to weight ratio when I am able to. Meanwhile, depth percentage needs to be used as a screening tool for spread comparison. It isn''t perfect, but it works a good deal of the time.
So, a princess cut with 60% depth will look larger than a princess cut with 71.5% depth? NO. Not necessarily. And it is the duty of a gemologist to point at the exceptions, which do not follow the ''general rule''. Or, would you say that all sapphires are treated, just because it is like tihis a good deal of the time?

Here is the simple example:

stone 1

Weight: 1.01
Depth: 60%
Diameter: 5.76 x 5.61

stone 2

Weight: 1.04
Depth: 71.5%
Diameter: 5.71 x 5.65

If a huge difference in depth of more than 10% does not necessarily give you any info on spread, then a smaller difference definitely does not give you the info.

The thing is: this is not only essential to understand, the fact that many people still think that less depth translates in more spread is one of the major causes why many fancy shapes are cut to worse looking proportions than they could have had.

Live long,
 

strmrdr

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To throw some more mud in the water and make a good point...

With rounds,,,,
To be very honest I don't care that much about depth or spread its not the right way to evaluate them.
Get the crown and pavilion angles needed for the highest performance combined with the minor facets and a standard girdle of acceptable depth to get the personality you want in a diamond.
Doing that the depth and spread take care of themselves.
They don't matter much.

You will get a much much better diamond that way than buying by spread or depth.
 
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