Symmetry and Polish standards

[Garry H (Cut Nut)]

I had to do this for the local diamond course lecture coming up this week.
I thought it might be interesting data for the folk here.

I dont think I have any old comparison data - but imaging the situation is much improved.
This is for all labs rounds only.
Of course fancies would not be as nice a looking set of data.

 

[strmrdr]

Interesting, thanks Garry

 

[kenny]

Garry, what do the numbers mean or represent?

How do they arrive at those numbers?

 

[Garry H (Cut Nut)]

Date: 9/14/2009 1:49:21 AM
Author: kenny
Garry, what do the numbers mean or represent?

How do they arrive at those numbers?

In the advanced search options I checked each one of the factors and that was the number of stones that were listed withing the size color clarity.
I did 10 searches

 

[diagem]

Date: 9/14/2009 2:46:02 AM
Author: Garry H (Cut Nut)

Date: 9/14/2009 1:49:21 AM
Author: kenny
Garry, what do the numbers mean or represent?

How do they arrive at those numbers?

In the advanced search options I checked each one of the factors and that was the number of stones that were listed withing the size color clarity.
I did 10 searches

Interesting and thanks...
By who were these graded? Or just randomly?

 

[Garry H (Cut Nut)]

Date: 9/14/2009 2:54:37 AM
Author: DiaGem

Date: 9/14/2009 2:46:02 AM
Author: Garry H (Cut Nut)

Date: 9/14/2009 1:49:21 AM
Author: kenny
Garry, what do the numbers mean or represent?

How do they arrive at those numbers?

In the advanced search options I checked each one of the factors and that was the number of stones that were listed withing the size color clarity.
I did 10 searches

Interesting and thanks...
By who were these graded? Or just randomly?

DG the vast majority of stones on Rapnet are GIA graded. EGL is next.

 

[Paul-Antwerp]

I do not think that this situation is way different than 5 years ago, maybe even 10 years ago, but does that matter?

Looking at these data, we could judge that VG and EX are plentiful in supply, and not extremely difficult to achieve.

Should the advice to consumers asking about a Good in polish and symmetry then not be: ''Well, it is a factor that not necessarily has a negative impact on the visual performance of the stone, but considering that 80% of the diamonds cut have a grade of VG or better, why would you think about buying a minor grade unless it carries a hefty discount.''

Just an idea.

Live long,

 

[Rockdiamond]

Hi All!
Garry- very interesting subject- thanks for starting it and doing the research!

In my experience there is a large difference from say, 20 years ago- and the difference is for the better.
Places like India, and Israel that were known for sub-standard makes ( poor cutting) now routinely manufacture really well cut diamonds. The development of factories in places like China, and Thailand have also created more well cut stones in the market in the smaller sizes.

Add to this the larger percentage of cutters who find value in taking the additional steps to get VG, or EX polish or symmetry, and the overall effect is that the "average" diamond is far better cut today as compared to 20 years back.

In the ''80''s, "Russian Makes" were considered the world''s finest. And IMO, Russian cut goods were head and shoulders above much of what was cut elsewhere.
Today there''s a lot of competition for that title.

 

[diamondbuyer52]

Gary,

I have a question for you; since you are the creator of the holloway cut advisor. I have been looking into purchasing a diamond. I showed my jeweler your HCA and he somehow converted the % crown and pavillion into degrees. Plugging in the following information below resulted in fair to good results on your HCA, but once my jeweler supposedly converted this to angles it showed all excellent and vg spread. First, is this a good valued diamond based off the varables below? And secondly, is there any conversion tables between % crown to degrees (angle) and % pavillion to degrees (angle) if so where can I find it? Lastly, does my jewelers conversions appear to be accurate? Please respond if possible.

Certification: EGL-USA
Dimensions: 7.27 - 7.23 x 4.49 mm
Carat: 1.46
Color: F
Clarity: VS2
Cut: Excellent (Tarkowsky Ideal - Hearts and arrows)
Polish: Very Good
Symmetry: Very Good
Depth: 61.9%
Table: 56%
Crown: 16% -------------> My jeweler changed to 34 degrees
Pavillion: 43% --------------> "( )" to 40.5 degrees
Culet: none
Price: $10,500.00

Thanks for your Help!!!

 

[jet2ks]

diamondbuyer52,

You will be better off starting your own thread instead of hijacking this one. There are several people on PS that can help you with that conversion, but they are more likely to see it if in its own thread.

 

[stone-cold11]

Date: 9/16/2009 11:31:55 PM
Author: diamondbuyer52
Gary,

I have a question for you; since you are the creator of the holloway cut advisor. I have been looking into purchasing a diamond. I showed my jeweler your HCA and he somehow converted the % crown and pavillion into degrees. Plugging in the following information below resulted in fair to good results on your HCA, but once my jeweler supposedly converted this to angles it showed all excellent and vg spread. First, is this a good valued diamond based off the varables below? And secondly, is there any conversion tables between % crown to degrees (angle) and % pavillion to degrees (angle) if so where can I find it? Lastly, does my jewelers conversions appear to be accurate? Please respond if possible.

Certification: EGL-USA

Dimensions: 7.27 - 7.23 x 4.49 mm

Carat: 1.46

Color: F

Clarity: VS2

Cut: Excellent (Tarkowsky Ideal - Hearts and arrows)

Polish: Very Good

Symmetry: Very Good

Depth: 61.9%

Table: 56%

Crown: 16% -------------> My jeweler changed to 34 degrees

Pavillion: 43% --------------> ''( )'' to 40.5 degrees

Culet: none

Price: $10,500.00

Thanks for your Help!!!

Your jeweler fails in basic geometry. This is just simple trigometry, which the HCA does to convert the % depth back to angles that is used to tabulate the HCA data points.

For the angles, you will need the tangent formula, tan(degrees) = opp/adj

Opp is the height of the right angle triangle, Adj is the base length of he right angle triangle.

Adj for the crown is (Total Diameter - Table Size)/2, since the numbers are all a percentage of the Diameter of the stone, that works out to,

Adj = (100 - 56) /2 = 22%

Opp = 16%

ArcTan(Opp/Adj) = ArcTan(16/22) = 36degrees

For the pavilion

Adj = 50%

Opp = 43%

ArcTan(Opp/Adj) = ArcTan(43/50) = 40.7degrees

Which is what is plotted on the HCA.