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starryeyed

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Question - when I do a search for "In-House" asschers, the results have additonal columns for L/W ratio and Spread. I understand L/W, but not the Spread number. I''ve seen the number range from 3-4% (positive) to -20% (negative).

What does this mean? What is this measuring for asschers? What is desirable? Is it better to be positive, negative, close to 0, far from 0?
 

strmrdr

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Date: 12/9/2006 10:31:54 PM
Author: starryeyed
Question - when I do a search for ''In-House'' asschers, the results have additonal columns for L/W ratio and Spread. I understand L/W, but not the Spread number. I''ve seen the number range from 3-4% (positive) to -20% (negative).

What does this mean? What is this measuring for asschers? What is desirable? Is it better to be positive, negative, close to 0, far from 0?
There is no standard for spread with asschers so ignore it.
check the total depth and if its under 70 kewl if its over 70 id want a discount.
 

Garry H (Cut Nut)

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Date: 12/9/2006 10:31:54 PM
Author: starryeyed
Question - when I do a search for ''In-House'' asschers, the results have additonal columns for L/W ratio and Spread. I understand L/W, but not the Spread number. I''ve seen the number range from 3-4% (positive) to -20% (negative).

What does this mean? What is this measuring for asschers? What is desirable? Is it better to be positive, negative, close to 0, far from 0?
Good question.
0% = the same millimeter squared surface area as a Tolkowsky round T53% C34.5 p40.75 and a thin to medium girdle (1% valley 2.6% main)
 

Garry H (Cut Nut)

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Date: 12/9/2006 11:22:11 PM
Author: strmrdr

Date: 12/9/2006 10:31:54 PM
Author: starryeyed
Question - when I do a search for ''In-House'' asschers, the results have additonal columns for L/W ratio and Spread. I understand L/W, but not the Spread number. I''ve seen the number range from 3-4% (positive) to -20% (negative).

What does this mean? What is this measuring for asschers? What is desirable? Is it better to be positive, negative, close to 0, far from 0?
There is no standard for spread with asschers so ignore it.
check the total depth and if its under 70 kewl if its over 70 id want a discount.
Storm you would buy a 65% depth with a -50% spread?

That is a nonsense approach.

In my experniance the spread of asscher is very important to consumers.

And this is the best measure of it. I use it always when buying diamonds.

(Warning; I had some involvement in designing the system so I am biased.)
 

strmrdr

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Date: 12/9/2006 11:28:15 PM
Author: Garry H (Cut Nut)

Date: 12/9/2006 11:22:11 PM
Author: strmrdr


Date: 12/9/2006 10:31:54 PM
Author: starryeyed
Question - when I do a search for ''In-House'' asschers, the results have additonal columns for L/W ratio and Spread. I understand L/W, but not the Spread number. I''ve seen the number range from 3-4% (positive) to -20% (negative).

What does this mean? What is this measuring for asschers? What is desirable? Is it better to be positive, negative, close to 0, far from 0?
There is no standard for spread with asschers so ignore it.
check the total depth and if its under 70 kewl if its over 70 id want a discount.
Storm you would buy a 65% depth with a -50% spread?

That is a nonsense approach.

In my experniance the spread of asscher is very important to consumers.

And this is the best measure of it. I use it always when buying diamonds.

(Warning; I had some involvement in designing the system so I am biased.)
Such a stone would be regected with regular pictures by me and a lot of other consumers who have looked at a lot of asscher pictures and by anyone with an aset.
That fat a pavilion will not return much of any light.
The actual mm demensions is important yes but but a big woofer is still a woofer.
 

starryeyed

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You guys have such a healthy debate going on, it''s great!

So if I were to assume that the Spread value is useful, what would I be looking for with Asschers? Any rules of thumb here?
 

strmrdr

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Date: 12/9/2006 11:59:43 PM
Author: starryeyed
You guys have such a healthy debate going on, it''s great!

So if I were to assume that the Spread value is useful, what would I be looking for with Asschers? Any rules of thumb here?
yea rule of thumb put:
great patterns
square
high crown
eyeclean
more important than spread. :}

Notice #3 there goes against spread.
 

Garry H (Cut Nut)

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actually I just noticed there is an error in the asscher spread calculations - it assumes the stone has its corners.

Will ask Leonid to assume 18% corners and reset the calculation.

In general nice asschers have a smaller spread than princess because they have more crown. But all things being equal a better spread makes the stone more desirable. That Storm is undenialble.

For an answer - Kenny''s asscher is -21% which is about middle of the road in my experiance (limitied). To get under -10% you would need a dangerously thin girdle.
 

kenny

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I am trying to follow.

What is -21% a measurement of?
 

strmrdr

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Garry, I don''t think we really disagree on spread I''m saying that it has too be kept in perspective and its down my list behind other things I consider more important.

If spread is high on someones list of desired attributes then an asscher is not the stone they should be looking at.
 

starryeyed

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So this diamond from JA has a positive spread value of 4%:

3.09 F VS1 61% Table, 70.9% Depth

Whereas this one from GOG is negative 19%:

2.01 F VS2 62% Table, 72.9% Depth

and this one from JA is -17%:

2.32 E VVS2 63% Table, 68.3% Depth

I don''t really understand this.....

Also, Garry, I saw a few with spreads less that 10% that have other than "thin" or "very thin" girdles. Here''s one from GOG that has a spread of negative 5%, but has a girdle of 3.71%.

2.03 E VS2, 60.7% Table, 66.2% Depth

I''m pretty confused by this....
 

kenny

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I read spread explanation in this thread again.

Let me put it in my words to see if I have it correct.
Asscher spread (as defined in Pricescope in-house search) is the face up area of the asscher (measured in square mm) compared to the face up area of a Tolkowski round of the same weight.

So my 2.26 carat assher has 21% less face up surface area than a 2.26 carat round with Tolkowsky measurements.
(Actually it will be less than 21% when the formula is corrected to take into account that the corners are chopped off of asschers.)

Right?


Next, Garry posted this:
"0% = the same millimeter squared surface area as a Tolkowsky round T53% C34.5 p40.75 and a thin to medium girdle (1% valley 2.6% main)"

I know the formula for the surface area of a circle is Area=3.14 x radius x radius.

But what is the forumula for calculating the face-up surface area of a Tolkowsky round for a given weight?
 

starryeyed

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Hi Kenny. I can do a basic derivation, based on the info from this thread:

r = avg. radius, in mm
c = carat weight
a = avg. diameter = 2r, in mm
d = depth, in mm

From the GIA formula:
Avg Diameter * Avg Diameter * Depth * 0.0061 = Carat Weight
a * a * d * 0.0061 = c

Substituting:
2r * 2r * d * 0.0061 = c

Multiplying:
0.0244r^2 * d = c

Solving for Radius Squared:
r^2 = 40.9836c/d

The area formula:
Area = 3.1416 * r^2

Substituting:
Area = 128.7538c/d

Therefore, if the face-up surface area is measured along one plane, the Area can be determined by the above formula, based on carat weight and depth. (I assume this is for Tolkowsky Ideal proportions only and that it would change for different crown/pavilion angles, etc)

If the face-up surface area is measured in two dimensions, i.e. - along the face of the "cone" portion of the crown, we'd have to re-figure. Maybe it's more complicated than this, but this is my best guess.
 

Garry H (Cut Nut)

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Date: 12/10/2006 11:00:50 AM
Author: starryeyed
So this diamond from JA has a positive spread value of 4%:

3.09 F VS1 61% Table, 70.9% Depth

Whereas this one from GOG is negative 19%:

2.01 F VS2 62% Table, 72.9% Depth

and this one from JA is -17%:

2.32 E VVS2 63% Table, 68.3% Depth

I don''t really understand this.....

Also, Garry, I saw a few with spreads less that 10% that have other than ''thin'' or ''very thin'' girdles. Here''s one from GOG that has a spread of negative 5%, but has a girdle of 3.71%.

2.03 E VS2, 60.7% Table, 66.2% Depth

I''m pretty confused by this....
That is why I pounced on Storm for recomending depth % as a measure of spread.

It is a blunt and dumb instrument

That is why Leonid and I added spread which tells more.

Although I consider another measure - verticle spread, or entire crown surface area as another usefull guide.
 

starryeyed

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Yes Garry, I would imagine that Spread depends on a lot of variables for an Asscher - all the pavilion angles, the crown angle, the amount of clip to the corners...in addition to the depth.

I guess that why I was wondering if there were any useful rules of thumb for spread - I though it might tell me a lot more about the cut of a stone without actually seeing it.

Like for 3-step asschers, for depths from m to n, and tables from p to q, spreads of r to s are best....

That kind of thing. Perhaps all the spread really answers for me is how small the thing looks!
 

kenny

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Yes, spread is face-up surface area for weight.

That's all spread is.

Low spread means it looks small for the weight.
High spread means it looks large for the weight.

Also I still am wondering how you convert weight to surface area in a round with Tolkowsky proportions.
Starryeyed, I don't think you answered this question.
 

strmrdr

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Date: 12/10/2006 2:40:45 PM
Author: starryeyed
Yes Garry, I would imagine that Spread depends on a lot of variables for an Asscher - all the pavilion angles, the crown angle, the amount of clip to the corners...in addition to the depth.

I guess that why I was wondering if there were any useful rules of thumb for spread - I though it might tell me a lot more about the cut of a stone without actually seeing it.

Like for 3-step asschers, for depths from m to n, and tables from p to q, spreads of r to s are best....

That kind of thing. Perhaps all the spread really answers for me is how small the thing looks!
The current numbers take none of that into account and cant as far as the PS database goes because the info isnt there.
Garry said above that they are bad numbers and he was going to fix it by assuming 18% corners.
18 % corners are just getting into the good zone and are done for weight retention, do we really want to encourage them?
I prefere 22-26 myself.

edit: changed degrees to % cuz I had a thinko as Garry pointed out below, thanks fior catching that.
 

strmrdr

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My main problem with it is that it throws another number into the waters that dont tell you much about the asscher itself.
The numbers are less than 20% of what you need to know.
How square and the crown height are 90% of that 20%.

Given 2 equaly well cut asschers of the same clarity and color and close in weight then spread might be a tie breaker to me.
But it is down the list of what is important.
 

biblobaggins23

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holy moly! wow, i had no idea buying diamonds was so scientific! thanks for all your answers!
 

Garry H (Cut Nut)

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Date: 12/10/2006 4:21:33 PM
Author: strmrdr

Date: 12/10/2006 2:40:45 PM
Author: starryeyed
Yes Garry, I would imagine that Spread depends on a lot of variables for an Asscher - all the pavilion angles, the crown angle, the amount of clip to the corners...in addition to the depth.

I guess that why I was wondering if there were any useful rules of thumb for spread - I though it might tell me a lot more about the cut of a stone without actually seeing it.

Like for 3-step asschers, for depths from m to n, and tables from p to q, spreads of r to s are best....

That kind of thing. Perhaps all the spread really answers for me is how small the thing looks!
The current numbers take none of that into account and cant as far as the PS database goes because the info isnt there.
Garry said above that they are bad numbers and he was going to fix it by assuming 18 degree corners.
18 degree corners are just getting into the good zone and are done for weight retention, do we really want to encourage them?
I prefere 22-26 myself.
Not true - the current Pscope spread score still gives a much better relativity than depth %.

Then if the corners are bigger then you can allow they will have a smaller spread and vice a versa
 

starryeyed

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Date: 12/10/2006 4:19:08 PM
Author: kenny

Also I still am wondering how you convert weight to surface area in a round with Tolkowsky proportions.

Starryeyed, I don''t think you answered this question.

I tried my best to derive an answer for you Kenny. Here is the formula again:

Area = 128.7538*c/d, where

Area = face-up surface area, in square mm
c = carat weight
d = depth, in mm

The constant 128.7538 is derived from the multiplier from the GIA formula that I previously linked. It is this GIA multiplier that probably takes into account crown/pavilion angles, etc. I woul imagine that if the diamond is different than the specific Tolkowsky proportions, the multiplier would change slightly. I''m guessing anyway.

I''ve tried this formula with a few specs and it works, but there may be limitations...
 

kenny

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I just grabbed an in-house asscher in PS look up.
1.01 carat G vvs2 5.77 x 5.64 x 3.79mm, spread -3%

(128.75 x 1.01) / 3.79 = 34.3 sq. mm

Now, were did they get the -3% spread from?

I assume 34.3 sq mm is 3% smaller than a tolkowsky of 1.01 carat.

Still.
Once again.
How do I calculate the face up area of a round with tolkowsky proportions?

Without this I still don't know how they arrived at -3%.
 

starryeyed

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Kenny, I''m not a diamond expert and I don''t know how the "Spread" calc is done exactly - if it''s 3% of the area or 3% of the diameter or what. I''d love to know that too.

I think the point that is confusing is that I believe the 34.3 sq mm would be the face-up surface area for that Tolkowsky ideal cut stone (assuming that what the GIAcalc is based on). The stone you grabbed probably doesn''t have the same proportions, so it would have a different multiplier and the formula doesn''t apply exactly. There''s probably a more complex formula with crown/pavilion angles, etc. because I''m guessing it depends on these variables also.

For the 34.3 sq mm face area, the average diameter would be roughly 6.61 mm.
 

kenny

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Sorry that stone I grabbed was an asscher.

I''ll edit my post.
 

starryeyed

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Yes, I agree with you Kenny that it''s tough to understand what the -3% spread means exactly.

The 34.3 sq mm is the face-up area of the Tolkowsky round (assuming that''s what the GIA used in their calc).

If the asscher spread is -3%, I''m not sure if that means the face-up area is smaller by 3%, or if the diameter is 3% smaller, or (from what Garry said) if the 3% is even correct because of the corner-issue.

We know the Asscher looks smaller face-up compared to an ideal RB, but understanding the spread calc comparison would help immensely!
 

Garry H (Cut Nut)

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what is so hard to understand *Eye?

Date: 12/10/2006 9:44:15 PM
Author: starryeyed
Yes, I agree with you Kenny that it''s tough to understand what the -3% spread means exactly.

The 34.3 sq mm is the face-up area of the Tolkowsky round (assuming that''s what the GIA used in their calc). i am totally unaware that GIA ever bothered to teach about spread - this is not a gIA thing it is a DiamCalc and Pricescope thing (that AGS picked up and used based on the DiamCalc standard of 6.47mm for a 1ct round)

If the asscher spread is -3%, I''m not sure if that means the face-up area is smaller by 3%, or if the diameter is 3% smaller, or (from what Garry said) if the 3% is even correct because of the corner-issue.
-3% is 3% smaller and +3% is 3% bigger than standard Tolkowsky round

We know the Asscher looks smaller face-up compared to an ideal RB, but understanding the spread calc comparison would help immensely! What is difficult to understand? Do you want the comparison math? It is mm squared for stone being examined divided by mm squared for Tolk round of the same carat weight converted to postive percentage (if bigger than 100%) or negative (if less than 100%)
 

strmrdr

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Garry reading this thread, using spread is throwing another near meaningless complication into an already complicated and hard to understand cut.
With rounds and square princess cuts its ok because they are all the same shape but it dont work with asschers.
There is no standard outside shape to apply a spread grade too.
 

starryeyed

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Hi Garry. Thanks for responding - you are far more experienced and knowledgeable than I am.

You''re right, I couldn''t find anything about "Spread" with GIA either. I could only find the equations I referenced.

The comparison math helps. An area that varies by 3% is different than a diameter varying by 3%. I understand now that spread is figured on area. Thanks!

I just hope Kenny is clear?
 

kenny

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I give up.
 
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