Why depth is not the opposite of spread in a fancy shape?

[Paul-Antwerp]

This subject is a side-topic that came up in this thread, and probably deserves its own thread.

My point is that one simply cannot say that a deeper fancy shape means that the stone has less spread. This is because of two reasons.

First, in many fancy shapes, there is the length-to-width-ratio. The depth is calculated on the basis of the smallest diameter, the width. So, two stones might have the same depth, but a L/W-ratio that is 15% different. (1.75 is almost 17% more than 1.50). So, the same depth could easily have a difference in spread of 15%.

Second, most fancy shapes do not have just one main crown and pavilion angle. Any stone with multiple crown- and pavilion-angles can hide weight without affecting the spread. I have written articles about this effect in the PS-journal on princess-cuts, but it is even more obvious in step-cuts. Again here, a conservative estimate is that the same depth can hide another 15% in weight in the bulge of the pavilion and the crown.

Taking both into consideration, it shows that two fancy shapes with the same depth-percentage can differ up to 30% in spread.

Considering that the difference between a depth of 60% and one of 78% is also 30%, how can one then use depth to say something about spread?

Live long,

 

[oldminer]

Paul;

Take a 2 pear shapes with the same outline and same length and width. The one with a 60% depth will weigh less than the one with a 65% depth. If a second system is used to show light performance and one is proved to be more brilliant and sparkly than the other, then the customer can choose which stone to buy knowing all the details. They will know why one weighs more and why one weighs less. They will know they both have the same visual size. Finally, they will know which one has more light performance. The diamond with less depth percentage has greater "spread" in the way I understand and use this term.

They might choose either stone and that is up to them, not to you or I.

Is one diamond a bigger show? Yes, the 60% depth one.
Is one a better performer? We don't know and can't tell from the depth.
Is one a better value? We don't know becuase we don't have prices, color, clarity, weight to make such a judgement.

So, the results of parametric analysis useful and limited, but not 100% incorrect. I have always said the results are limited and have to do with what we have called craftsmanship issues, not beauty selection.

 

[kenny]

I thought spread was how big it looks (when viewed straight down) for the weight.
How big it looks is more accurately described by its area, not just width and length.

Area is easy to calculate for a perfect square or rectangle; it's length x width.
Area of a perfect circle is 3.14 times the square of the radius. (pie R squared)

But once you chop off the corners of a square or rectangle or elongate that circle or change it to a pear shape the area becomes more complex to calculate, and it messes with the brain (our perception).

Also, besides the math, perception of area may no longer match the mathematical area.
Even though the corners are missing from an emerald cut, the brain may fill it in giving the impression of a full rectangle.
A perfectly square 6mm princess cut next to a 6mm round may seem more similar in area than they really are mathematically.
A marquise looks huge for its weight.

Shapes play tricks on the brain.

 

[HopeDream]

Fascinating!

 

[Lorelei]

I think this is a great idea for a thread Paul, this definitely needs addressing, thank you for bringing it up!

 

[Karl_K]

There are 2 different concepts here:

Absolute spread which is lxw

Apparent spread is not as cut and dried.
Apparent spread can be impacted by outline, side to side brilliance, somewhat table size and crown height if strait up is the only consideration, slightly tilted the 3d effect can and does make higher crowns and small tables look larger but they can look smaller strait up in some designs but not in others.

Neither of these is directly related to depth in a step cut and many fancies.
I will post some examples later today.

 

[Karl_K]

These 2 have the same depth and LxW but one weighs 1.04ct and one weighs 1.15ct.
If spread was proportional to depth then they would weigh the same.
Ok that is cheating a little, but later I will show samples with the same corners.

 

[Karl_K]

These 2 every number on a sarin report would be the same but it is obvious one would weigh more.

 

[kenny]

Date: 3/26/2010 1:04:09 PM
Author: Karl_K
These 2 every number on a sarin report would be the same but it is obvious one would weigh more.

Sorry Karl, it''s not obvious to me.
The only difference I spot is the lowest horizontal line is in a different place.
Could you explain please how that would change the weight?

 

[ChunkyCushionLover]

Date: 3/26/2010 11:36:22 AM
Author:Paul-Antwerp
This subject is a side-topic that came up in this thread, and probably deserves its own thread.

My point is that one simply cannot say that a deeper fancy shape means that the stone has less spread. This is because of two reasons.

First, in many fancy shapes, there is the length-to-width-ratio. The depth is calculated on the basis of the smallest diameter, the width. So, two stones might have the same depth, but a L/W-ratio that is 15% different. (1.75 is almost 17% more than 1.50). So, the same depth could easily have a difference in spread of 15%.

Second, most fancy shapes do not have just one main crown and pavilion angle. Any stone with multiple crown- and pavilion-angles can hide weight without affecting the spread. I have written articles about this effect in the PS-journal on princess-cuts, but it is even more obvious in step-cuts. Again here, a conservative estimate is that the same depth can hide another 15% in weight in the bulge of the pavilion and the crown.

Taking both into consideration, it shows that two fancy shapes with the same depth-percentage can differ up to 30% in spread.

Considering that the difference between a depth of 60% and one of 78% is also 30%, how can one then use depth to say something about spread?

Live long,

The matter of depth in a princess cut from PS Journal

Numbers and perception, the case of square cut diamonds from PS Journal

Paul,

I wrote something in this thread which I had hoped you would read. Please address it here if you wouldn't mind I mostly cut and pasted was wondering what you thought.


I really liked your commentary on spread in princess cuts the conclusions were valid see links above, however the calculation method is inconsistant from one paper to the next and I think there is a simpler and more accurate ways of comparing spread in a round to a princess without using "Math Wizardry".

In the first article link Paul calculated a depth of a princess as 62.13% specifically 4.5/(8.19 + 6)/2. Or Depth / Average of (Diagonal and Length) in the second article he used

Depth / Average of (Length and Width) (He didn't use diagonal in this example and came up with a higher depth percentage) (Example 2)
-----------------------------------------------------------------------------

Neither calculation methods are accurate for comparing depth percentages for the purposes of a spread/carat weight comparison between a round and a square shape, however the method in the second link is more consistant with that of round measurements and it would be preferred.

The problem with the average diameter in a round calculation is that it isn't truly a weighted average of all diameters of the stones it only considers two distances, whereas it really should consider all diameters. This is a reasonable assumption to save time however because most round diamonds are really close to being perfectly symmetrical so taking the longest point and 45 degrees from this point is a reasonable assumption.

However doing this for a square or rectangular shape is more incorrect as the diameters can be quite different depending on which points are taken and the weighted average of all diameters does not equal the simple average of the length and width or any other two distances one may choose.

Instead of using average diameter I would much prefer a direct surface area comparison which is easy to do in very near round and very near square shapes.

SA (Princess) = Length * Width
SA (Round) = 3.14159 * (Average Diameter/2)*(Average Diameter/2)

If one uses surface area as a comparison one would find that on comparing two CBI diamonds

=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50:2o8h065d]http://www.highperformancediamonds.com/index.php?page=diamond-search&sort=&order=&offset=&scope=global&search=1&shapes[]=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50

=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50:2o8h065d]http://www.highperformancediamonds.com/index.php?page=diamond-search&sort=&order=&offset=&scope=global&search=1&shapes[]=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50

Round Diamond 1.2 Carats (Thin to Thick Girdle) Surface Area = 37.1 mm squared
Princess Cut 1.2 Carats (Thin to Medium Girdle) Surface Area = 32.78 mm squared

The princess in this example has 11% less spread than the round for the same carat weight.
However even other well cut princess diamonds like this WF ACA princess. http://www.whiteflash.com/aca_princess/whiteflash-aca-princess-cut-diamond-2270399.htm#

1.192 Carats has a Surface Area = 33.35 mm Squared.

So now even for a slightly lower carat weight the princess is only 10% less spread than the same carat weight round.

I feel that a direct comparison of girdle plain surface area will serve these comparisons much better than explaining the different ways of considering depth.

One could readily find using this method that other princess cuts, optimized for weight retention rather than light performance with more shallow crowns (Depths in the low 70% for example) the spread differences between a round and princess then becomes even less. The differences would also be smaller if I used a round with exact Tolk depth of 61.5% instead of one in this example at 61.2%

 

[ChunkyCushionLover]

Kenny,

The one on the right weighs more.
Adding height in P3 adds less weight than adding height in P2. Just in case I get the section naming wrong adding height in the triangular lower point adds less weight than adding height in the section above it.

The sarin report doesn't tell you the height of the steps

?
It should.

 

[kenny]

Thanks CCL.

 

[Karl_K]

Date: 3/26/2010 1:21:24 PM
Author: ChunkyCushionLover
Kenny,


The one on the right weighs more.

Adding height in P3 adds less weight than adding height in P2. Just in case I get the section naming wrong adding height in the triangular lower point adds less weight than adding height in the section above it.


The sarin report doesn't tell you the height of the steps

?

It should.

the reports where it lists the numbers does not.
The 3d viewer will give you the right outline however.

The biggest difference is how far the p1 goes down the pavilion.
That is where weight is commonly hidden in step cuts.
Steep and long p1

 

[oldminer]

One can show with the new technology of DiamCalc that the way a diamond is cut can contribute to extra retained wieght without increasing the depth. The "bulge" increases in the pavilion or possibly even in the crown and that can keep more weight in the diamond while not adding to its depth measure. What one cannot do is not add weight when such a bulge is put into the diamond design. Even when the depth doesn't change a comparison of girdle plane surface area against the weight of the diamond would reveal the greater weight per square mm of surface area when more weight is retained by some cutting strategy. One very neat thing about the ImaGem technology is that is supplies the exact girdle plane surface area of ANY diamond processed regardless of outline and shape. This technology counts the subpixels and since it is highly calibrated it can readily convert into millimeters and the surface area is no trick to obtain. In cases such as these with a bulge or no bulge, the depth measure is not related to spread.

The trade tradtiionally used the word "spready" to describe a thinnish diamond which had a largish visual size. It was not a technical term, but one that was apparent to any diamond guy and to most consumers, too.

In more technical terms we use "spread" to compare visual surface area of the girdle plane to the weight of a given diamond. Two diamonds having identical outlines and visual surface area with one weighing less than the other, one would say the spread is greater on the lighter stone. Do we agree that this would be one legitimate explanation for how dealers use "spread" in describing diamonds? Are there other definitions which would conflict with this one? How else would "spread" be defined?

 

[Paul-Antwerp]

Date: 3/26/2010 1:15:22 PM
Author: ChunkyCushionLover

Date: 3/26/2010 11:36:22 AM
Author:Paul-Antwerp
This subject is a side-topic that came up in this thread, and probably deserves its own thread.

My point is that one simply cannot say that a deeper fancy shape means that the stone has less spread. This is because of two reasons.

First, in many fancy shapes, there is the length-to-width-ratio. The depth is calculated on the basis of the smallest diameter, the width. So, two stones might have the same depth, but a L/W-ratio that is 15% different. (1.75 is almost 17% more than 1.50). So, the same depth could easily have a difference in spread of 15%.

Second, most fancy shapes do not have just one main crown and pavilion angle. Any stone with multiple crown- and pavilion-angles can hide weight without affecting the spread. I have written articles about this effect in the PS-journal on princess-cuts, but it is even more obvious in step-cuts. Again here, a conservative estimate is that the same depth can hide another 15% in weight in the bulge of the pavilion and the crown.

Taking both into consideration, it shows that two fancy shapes with the same depth-percentage can differ up to 30% in spread.

Considering that the difference between a depth of 60% and one of 78% is also 30%, how can one then use depth to say something about spread?

Live long,

The matter of depth in a princess cut from PS Journal


Numbers and perception, the case of square cut diamonds from PS Journal

Paul,

I wrote something in this thread which I had hoped you would read. Please address it here if you wouldn''t mind I mostly cut and pasted was wondering what you thought.


I really liked your commentary on spread in princess cuts the conclusions were valid see links above, however the calculation method is inconsistant from one paper to the next and I think there is a simpler and more accurate ways of comparing spread in a round to a princess without using ''Math Wizardry''.


In the first article link Paul calculated a depth of a princess as 62.13% specifically 4.5/(8.19 + 6)/2. Or Depth / Average of (Diagonal and Length) in the second article he used

Depth / Average of (Length and Width) (He didn''t use diagonal in this example and came up with a higher depth percentage) (Example 2)
-----------------------------------------------------------------------------

Neither calculation methods are accurate for comparing depth percentages for the purposes of a spread/carat weight comparison between a round and a square shape, however the method in the second link is more consistant with that of round measurements and it would be preferred.

The problem with the average diameter in a round calculation is that it isn''t truly a weighted average of all diameters of the stones it only considers two distances, whereas it really should consider all diameters. This is a reasonable assumption to save time however because most round diamonds are really close to being perfectly symmetrical so taking the longest point and 45 degrees from this point is a reasonable assumption.

However doing this for a square or rectangular shape is more incorrect as the diameters can be quite different depending on which points are taken and the weighted average of all diameters does not equal the simple average of the length and width or any other two distances one may choose.

Instead of using average diameter I would much prefer a direct surface area comparison which is easy to do in very near round and very near square shapes.

SA (Princess) = Length * Width
SA (Round) = 3.14159 * (Average Diameter/2)*(Average Diameter/2)

If one uses surface area as a comparison one would find that on comparing two CBI diamonds

=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50:2o8h065d]http://www.highperformancediamonds.com/index.php?page=diamond-search&sort=&order=&offset=&scope=global&search=1&shapes[]=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50

=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50:2o8h065d]http://www.highperformancediamonds.com/index.php?page=diamond-search&sort=&order=&offset=&scope=global&search=1&shapes[]=1&shapes[]=2&price_from=0&price_to=200,000&carat_from=1.2&carat_to=1.2&color_grade_from=1&color_grade_to=24&clarity_grade_from=12&clarity_grade_to=10&show_infinity=1&show_cut_grade[]=1&show_cut_grade[]=2&show_cut_grade[]=3&show_cut_grade[]=All&Submit=++++Search++++&limit=50

Round Diamond 1.2 Carats (Thin to Thick Girdle) Surface Area = 37.1 mm squared
Princess Cut 1.2 Carats (Thin to Medium Girdle) Surface Area = 32.78 mm squared

The princess in this example has 11% less spread than the round for the same carat weight.
However even other well cut princess diamonds like this WF ACA princess. http://www.whiteflash.com/aca_princess/whiteflash-aca-princess-cut-diamond-2270399.htm#

1.192 Carats has a Surface Area = 33.35 mm Squared.

So now even for a slightly lower carat weight the princess is only 10% less spread than the same carat weight round.

I feel that a direct comparison of girdle plain surface area will serve these comparisons much better than explaining the different ways of considering depth.

One could readily find using this method that other princess cuts, optimized for weight retention rather than light performance with more shallow crowns (Depths in the low 70% for example) the spread differences between a round and princess then becomes even less. The differences would also be smaller if I used a round with exact Tolk depth of 61.5% instead of one in this example at 61.2%

CCL,

The articles were not intended to launch a new definition of depth-measurement. They were intended to show how various existing calculation-methods give highly different, possibly psyschologically different, resulting depth-percentages.

In one of the articles, I clearly show that the calculation-method, traditionally used gives a very high percentage. If the same method were used as that of rounds, the percentage would be a lot different. And then I show how HOF uses a totally different method for its Dream-diamond, resulting in an even lower depth-percentage. The latter is probably based upon commercial reasons. If you research some old threads, you will definitely find a lot of reactions to the Dream-diamond as it being a very spready stone for a round with such low depth.

I fully agree that the surface of the girdle-plane would be a measure for depth, but that was never the purpose of the articles.

Live long,

 

[Paul-Antwerp]

Date: 3/26/2010 11:49:37 AM
Author: oldminer
Paul;

Take a 2 pear shapes with the same outline and same length and width. The one with a 60% depth will weigh less than the one with a 65% depth. If a second system is used to show light performance and one is proved to be more brilliant and sparkly than the other, then the customer can choose which stone to buy knowing all the details. They will know why one weighs more and why one weighs less. They will know they both have the same visual size. Finally, they will know which one has more light performance. The diamond with less depth percentage has greater ''spread'' in the way I understand and use this term.

They might choose either stone and that is up to them, not to you or I.

Is one diamond a bigger show? Yes, the 60% depth one.
Is one a better performer? We don''t know and can''t tell from the depth.
Is one a better value? We don''t know becuase we don''t have prices, color, clarity, weight to make such a judgement.

So, the results of parametric analysis useful and limited, but not 100% incorrect. I have always said the results are limited and have to do with what we have called craftsmanship issues, not beauty selection.

David,

Thank you for proving me right. Simply by limiting your comparison to pear-shapes (probably the fancy-shape with the least room to hide weight) the same outline (taking away one more option to hide weight) and the same length and width (taking away the first reason I mentioned in the original post), you are proving my point. The fact that you need to limit the case to such an extreme example, including completely neglecting half of my case (the half that is applicable to pear-shapes) is a confirmation of what I said.

Live long,

 

[oldminer]

Paul;

I truly did not go out of my way to use a special case with pear shapes. Sorry that you suspect the worst when all I wished to do was to give a clear example of how weight can be retained without an increase in depth percentage. I thought that this was your point and I accepted that you are correct in many cases. For whatever reason you say or imply that because we don''t totally agree you think that I am being less than honest or not telling the truth. I suppose you believe you have already proven your point of view 100%. WhileI believe I have given a convincing example of how spread and depth do not work hand in hand all the time.

Spread and depth sometimes function together and sometimes they don''t. Is there something wrong with this statement?

 

[Paul-Antwerp]

David,

Forgive me for not understanding what you are saying.

My point is that if I tell you that a certain diamond (fancy shape) has a depth of x %, nobody can make a definite statement regarding the spread of the stone, or about the stone having a small or big surface for its weight.

Do you agree or do you not agree?

 

[Regular Guy]

I''m betting this is relevant!

 

[oldminer]

Paul;

If you tell me only the depth% of a diamond I can't judge if it is spready or not in all cases. However, if you tell me a depth that is easily known as a shallow depth%, then one would naturally understand the stone must show a large surface area for its weight. We could not guess how much surface area results from a depth% given to us, but shallow depth does tend to generate larger spread. Deep depth% tells us nothing about spread, but a deep depth% does indicate that the diamond is holding weight in its depth and not tending to maximize visual size for its weight. A deep depth% stone would hardly ever be referred to as a spready stone and may have even more weight in a bulge effect if it is cut that way to even hold more weight and again, that would not be spready either.

Your argument that depth% does not equate to spread is correct, but not totally valid. My argument is correct in certain cases and not totally valid in all cases, either. This is why blind reliance on parameters is a faulty selection method, but useful for consumers to screen stones for possible purchase. They may needlessly eliiminate some good stones, but they will still end up with a good selection of stones from which they can choose in relative safety, too. Screening is useful and no one is arguing that screening tools are perfect.

 

[Paul-Antwerp]

Dave,

I have taken the weekend to read and re-read your post, and I do not understand it.

My point is that a fancy shape with a depth of 60% can have less spread than one with 75%.

Your point seems to be that this generally is not the case. Considering that diamond-cutters, even more in fancy-shapes, ''generally'' try to maintain weight wherever possible, you have a point.

The problem is that translating a correct observation into an incorrect ''rule'' that a higher depth-stone is holding weight in its depth and is not maximizing visual size for its weight, is becoming a self-fulfilling prophecy. It basically prevents cutters from actually cutting the deep-spready stone, since it automatically gets ''branded'' as too small for its weight.

As such, depth should never be used to judge the spread of a fancy-shape. Always, one should consider the actual diameters. And even then, one should be careful, since one could get misled by a novice notation-mode of diameters, like in the HOF-dream.

Live long,

 

[oldminer]

Quoting Paul: "My point is that a fancy shape with a depth of 60% can have less spread than one with 75%. "

Yes, this "can" be true, but it can also be false.

Quoting my own previous posting "Your argument that depth% does not equate to spread is correct, but not totally valid."

If you have two diamonds diamonds with the exact same outline shape, the same identical visual size, ie: identical square millimeter dimensions, and one is 60% deep and the other is 75% deep, the 75% deep one will weigh more and the 60% deep one will have more spread...............Do you understand now what I have been writing about? I don''t see this as a difficult message. Is there general confusion on this, or is it clear enough?

Spready diamonds weigh less for their visual size than deep stones. Deep stones weigh more for their visual size. Yes, you can hide additional weight in most any cut or depth diamond and the rule is not 100% true, but it is very useful when screening equal weight diamonds that have varying depths and dimensions. The best diamonds come in a depth range which is neither spready nor overly deep. Some diamonds are subject to further weight gain by bulge design faceting, too. Simple parametric charts can''t take bulge and deep design into account, but with the majority of diamonds the depth percentage can be quite useful in understanding how much visual size the stone will offer the viewer or if it will be thin enough to make it more watery in appearance.

 

[diagem]

Date: 3/29/2010 8:05:21 AM
Author: oldminer
Quoting Paul: ''My point is that a fancy shape with a depth of 60% can have less spread than one with 75%. ''

Yes, this ''can'' be true, but it can also be false.

Quoting my own previous posting ''Your argument that depth% does not equate to spread is correct, but not totally valid.''

If you have two diamonds diamonds with the exact same outline shape, the same identical visual size, ie: identical square millimeter dimensions, and one is 60% deep and the other is 75% deep, the 75% deep one will weigh more and the 60% deep one will have more spread...............Do you understand now what I have been writing about? I don''t see this as a difficult message. Is there general confusion on this, or is it clear enough?

Spready diamonds weigh less for their visual size than deep stones. Deep stones weigh more for their visual size. Yes, you can hide additional weight in most any cut or depth diamond and the rule is not 100% true, but it is very useful when screening equal weight diamonds that have varying depths and dimensions.

Useful when screening equal weight Diamonds that are cut to mainstream/generic faceting designs and proportions..., once you need to screen a novelty cut or a step-cut the chart becomes pretty much irrelevant.
Hiding weight is a ''general'' term when looking at proportion charts..., High crown height to compliment designs is IMO not considered hiding weight. Most step cuts or/and non-traditional 57/8 facet brilliant (eg Princess/Radiants etc...) fancy cuts do possess a pavilion bulge as part of the faceting design. The issue is when the bulge is exaggerated. And that is a problem with charts.

The best diamonds come in a depth range which is neither spready nor overly deep. Some diamonds are subject to further weight gain by bulge design faceting, too. Simple parametric charts can''t take bulge and deep design into account, but with the majority of diamonds the depth percentage can be quite useful in understanding how much visual size the stone will offer the viewer or if it will be thin enough to make it more watery in appearance.

Also too general..., best??
Its an axioma in the trade & basic educated consumer to limit depth to specific facet designs.
I witnessed many step-cut designs that were ruined by the 69.99% rule and/orl imit.
These limits sometimes force cutters to incorrectly complete a cut, I believe the 69.9% limit on step-cut designs is almost the kiss of death in many Diamonds.
It translates to me that the cut was forced to stay within the depth "norm" and usually points to error-ed completion.

As far as watery appearance..., it greatly depends on the crown design applied in conjunction with the shallow pavilion.
And secondly..., some actually look for the watery appearance and believe its beautiful..., we can notice this in finely cut shallow Antique Diamonds.
Diamonds like the Historic Hope or Wittelsbach are considered a type of watery appearance. Beautiful to my eyes

.
Some shallow standard fancy shapes like Pears or MQ''s that possess correctly applied 4 main pavilions combined with correctly applied crown heights & angles can be gorgeous as well & super spready

.

 

[Paul-Antwerp]

Date: 3/29/2010 8:05:21 AM
Author: oldminer
Quoting Paul: ''My point is that a fancy shape with a depth of 60% can have less spread than one with 75%. ''

Yes, this ''can'' be true, but it can also be false.

Quoting my own previous posting ''Your argument that depth% does not equate to spread is correct, but not totally valid.''

If you have two diamonds diamonds with the exact same outline shape, the same identical visual size, ie: identical square millimeter dimensions, and one is 60% deep and the other is 75% deep, the 75% deep one will weigh more and the 60% deep one will have more spread...............Do you understand now what I have been writing about? I don''t see this as a difficult message. Is there general confusion on this, or is it clear enough?

Spready diamonds weigh less for their visual size than deep stones. Deep stones weigh more for their visual size. Yes, you can hide additional weight in most any cut or depth diamond and the rule is not 100% true, but it is very useful when screening equal weight diamonds that have varying depths and dimensions. The best diamonds come in a depth range which is neither spready nor overly deep. Some diamonds are subject to further weight gain by bulge design faceting, too. Simple parametric charts can''t take bulge and deep design into account, but with the majority of diamonds the depth percentage can be quite useful in understanding how much visual size the stone will offer the viewer or if it will be thin enough to make it more watery in appearance.

Re-quoting your highlighted sentence: "If you have two diamonds diamonds with the exact same outline shape, the same identical visual size, ie: identical square millimeter dimensions, and one is 60% deep and the other is 75% deep, the 75% deep one will weigh more and the 60% deep one will have more spread"

This exactly is not true. It is not true for princess, nor for any step-cut, to only name those.

The fact that you are limiting it to the same outline-shape is an extra limiting factor, that one cannot see when considering depth as such. In all rectangular shapes, ovals and pears, the outline-shape changes the spread without affecting the depth.

Again, there is no necessary inverse relationship between depth and spread, and the problem is that anyone repeating that there is, is actually preventing the production of spready deep stones.

Live long,

 

[originalradiantman]

I haven''t posted here in a long time but this is a topic near and dear to my heart and a friend pointed it out to me. Paul''s point - that depth % doesn''t correlate to spread well enough in many fancy shapes to be a useful indicator - is 100% correct, for precisely the reasons he gives. That is why I only use real geometric surface area - Length x Width - area lost due to cut corners - to compare spreads in radiant cuts. In fact, with radiant cuts, I consider depth % calculated using only the width (and not the length) of the diamond, as GIA calculates it, to be a completely useless piece of information if it is not accopanied with alot of other data.

It is probably true that a fancy with a significantly higher depth percentage ( 5% +) is statistically more likely to spread smaller, but it need not be the case and small differences like 2-3% mean absolutely nothing probably not even yielding a statistically significant correlation to spread differences.

A metric that is that unreliable is not a metric at all. Using depth % as a proxy for spread in fancies will lead to the wrong answer in far too many cases for it to be a useful metric.

If I''m understanding Oldminer''s point correctly, and I''m not sure I am, he''s saying the metric is useful because its sometimes true. IMHO Paul is correct that it is untrue sufficiently often so as to make it virtually worthless as a means to compare spreads, except perhaps wihen the differences are very extreme.

 

[oldminer]

The GIA depth percentage metric for fancy shapes is pretty much a worthless measure, I agree. But, it is what is used in the trade and understood. When you relate depth to width only, as the GIA does, the metric does not mean much with many fancy shapes. Many of us would prefer to see the surface area contained within the girdle plane used as a metric of visual size so diamonds could be compared for how large they looked compared to one another.

I use spread to mean how the overall stone appears, not as just as depth relates to width. Spready means the visual size is large for the weight and Lumpy means the visual size is small for the weight. In the middle are the stones which are the best compromise of size and light return properties. Although Paul or another cutter can go out of their way to increase weight without increasing depth by creating a bulging pavilion, cutters can''t subtract weight to create a light weight stone with a large depth percentage as compared to a shallow depth percentage stone which will normally be larger visually for its weight than a similarly cut deeper percentage stone.

Beauty is another topic and not controlled by depth percentage. You can have lovely rather deep stones. However, it is a better deal for most people to find a beautiful stone that looks larger for the weight and still looks superb, at the same cost. That''s why I encourage less depth percentage, up to a reasonable point, for diamonds. I don''t promote shallow makes any more than overly deep ones. The ones in the middle zone have the right compromises of visual size and light return properties to make them safer choices for distant buyers.

I am sure Paul is right and can absolutely prove his case that there are instances of greater spread in relation to width than I might expect to ever see, but in the real world, I have found that deep depth diamonds, while possibly very pretty, generally give up something in visual size compared to competing diamonds of the same visual size and more moderate depth percentages. It is not 100% true in all cases, but we are discussing generalities.

The entire process would be improved if the trade were to provide girdle plane square millimeters for each diamond. Then everyone would know how to compare one diamond to another for visual size in a truly objective manner.

I know my concepts are in the minority camp and you guys can have the last say on this. I have covered it as best I can and don''t want to create the atmosphere of an argument because we respect eachother too much. Thanks for the opportunity of expressing my views.

 

[originalradiantman]

Oldminer - I think you hit the nail on the head when you pointed out that traditional depth % is useless but it''s what the industry knows and understands. The problem is that it is what the industry incorrectly thinks it understands, and that misunderstanding distorts the cutting process as well as misleads consumers. Pretending that depth% means something that it doesn''t, simply because its what many incorrectly believe, does a disservice to both the public and the industry.

If I wanted to I could cut easily cut a lumpy radiant cut with a low (60% or less) depth% by making a full top, high pavillion girdle breaks and a flat culet. Such a diamond could look great on paper - even be a 1A on your chart but it would look small and have terrible life. Using bad metrics because that''s what the trade has always done is not, in my opinion, an intellectually defensible choice if we know those metrics to be wrong.

In the absense of better information looking at depth% for what its worth makes sense, but only as long as we remember that its not worth much. I have often seen, on this forum and others, posters advise consumers that a fancy shaped diamond is "small" based solely on the fact that the depth% is higher than some believe it should be. I''ve seen that advice given when the actual measurements make clear that it is not true.

When we know a metric is wrong, it is, in my opinion, our obligation to say so, not to perpetuate the use of bad metrics because it is what the industry thinks it understands.

 

[Rockdiamond]

Thanks for posting Stan. You're an asset to the discussion.

 

[oldminer]

Stan; I agree with Rockdiamond that you are an asset to this discussion. In my thinking process I made the assumption that a cutter would never willingly create a diamond which turned out to look bad. Of course, we know that there are some pretty awful looking diamonds that have been cut, but when the rough allows, the cutter uses their skill to not only save weight, but to create beauty. Although I am certain you can prove the point you made by cutting such a strange radiant cut, I''d suspect you would rarely choose to do so and choose to make a far more beautiful stone. Since rough costs so much and beauty helps to make sales, I believe the market tends to create the best looking possible stone from each piece of rough. This need to make the diamond saleable combined with the inherent optical properties of diamond makes certain ranges of cutting necessary for each shape in order to make the stone look good and cost the right price.

If someone wants to prove that theoretically a diamond can have a strange combination of AGA 1 characterisitcs and still not look nice, I imagine it can be accomplished, but since it would be so costly to have that kind of diamond, cutters would not do it unless there were some extreme circumstance such as proving a point about the danger of pre-screening by parameters. In the real world, I don''t think this frequently happens, but your point is well taken and I agree with the majority here that a trusted, qualified vendor is the best partner for a diamond consumer. There are many vendors here who I believe consumers can trust to lead them in the right direction. For consumers who really don''t want to trust anyone, the problem still remains and we want them to find good help too.

 

[Garry H (Cut Nut)]

Date: 3/29/2010 5:08:58 PM
Author: oldminer

If someone wants to prove that theoretically a diamond can have a strange combination of AGA 1 characterisitcs and still not look nice, I imagine it can be accomplished,
I have taken the challenge Dave

I agree with the majority here that a trusted, qualified vendor is the best partner for a diamond consumer. There are many vendors here who I believe consumers can trust to lead them in the right direction.
The problem there is choosing which vendor to trust the most with which shape etc. I prefer consumer friendly systems to vendor based and innefficient shipping of goods hither and thither.

Gentlemen I am going to make Paul cross again with a ‘politicial’ pronouncement for a solution to this dilemma.

The image below shows an extreme example of two ovals, one cushionish, where the table and depth % are the same.
Same LXW, same gidrdle thickness, but look at the two weights and spread data in the lower right of each DiamCalc window.

AGS also adopted this spread approach that we developed for DiamCalc.
And for some inhouse stones we have adopted a similar approach on Pricescope (but it only works for a few sahpoes because we can not see the outline.

Sergey agrees that we should put this into information into Gem Adviser to protect consumers.

Next Diamond has a plan to calculate diamond values based on a simple (for Sergey and Janak) algorithm that uses spread and basic light return information to calculate a value (as always - compared to a Tolkowsky round). This would do away with the rather silly range of discounts that apply in the trade with round and pear shaped Rapaport price lists.

(This political announcement was authorized by the Peoples Consumer Rights Party)