Angle Dimensions / Measurements for ideal cut, great optical symmetry etc etc

[haagen_dazs]

Hi all
I am learning so much
I notice that there are alot of experienced folks on the forum that can tell if certain numbers are off by looking at the grading reports.
No doubt nothing is for certain until various images are seen.
I also know that there is no fix formula so I am just asking for a general idea.

This is what I know
Please correct me if i am wrong.

Table %
Smaller = generally more fire
Larger = more brilliant
Generally between 53% and 58% was desired

Pavilion
Between 75% and 85% desired

What about the crown angles to pavilion angle combos?\
What about the depth % ?

 

[Lorelei]

Here is the cheat sheet some of us use as a guide to find a well cut round diamond.

depth - 60 - 62% - although my personal preference is to allow up to 62.4%
table - 54- 57%
crown angle - 34- 35 degrees
pavilion angle - 40.6- 41 degrees
girdle - avoid extremes, look for thin to slightly thick, thin to medium etc
polish and symmetry - very good and above


note - with crown and pavilion angles at the shallower ends ( CA 34- PA 40.6) and steeper ( CA 35- PA 41) check to make sure these angles complement in that particular diamond - eyeballs, Idealscope, trusted vendor input - check as appropriate!

As the above implies, configurations depend on each other. A little give here can still work with a little take there.

From expert John Pollard.

With that said, here's a "Cliff's Notes" for staying near Tolkowsky/ideal angles with GIA reports (their numbers are rounded): A crown angle of 34.0, 34.5 or 35.0 is usually safe with a 40.8 pavilion angle. If pavilion angle = 40.6 lean toward a 34.5-35.0 crown. If pavilion angle = 41 lean toward a 34.0-34.5 crown.

GIA "EX" in cut is great at its heart, but it ranges a bit wider than some people prefer, particularly in deep combinations (pavilion > 41 with crown > 35).

 

[Lorelei]

Date: 9/17/2009 11:50:46 PM
Author:haagen_dazs
Hi all
I am learning so much
I notice that there are alot of experienced folks on the forum that can tell if certain numbers are off by looking at the grading reports.
No doubt nothing is for certain until various images are seen.
I also know that there is no fix formula so I am just asking for a general idea.

This is what I know
Please correct me if i am wrong.

Table %
Smaller = generally more fire
Larger = more brilliant
Generally between 53% and 58% was desired You are generally right, a smaller table coupled with a steeper crown angle can aid fire, conversely a larger table with a more shallow crown can equal more brilliance.

Pavilion
Between 75% and 85% desired This is the measurement for lower girdle facet percentages, yes as a rule of thumb those figures work. Stars - 50 - 60%.

What about the crown angles to pavilion angle combos?
What about the depth % ? For these see below.

Optical symmetry is different to lab graded physical symmetry. Optical symmetry is the study of the internal reflections of the diamond which are the result of great overall cutting precision, resulting in a hearts and arrows pattern. Only diamonds with top optical symmetry are deemed worthy of carrying the h&a brand names, these diamonds often have top proportions also.

Quoted from Jon at GOG.

"Well ... here's the Rhinocerotic "nutshell" version. Lab graded symmetry/ie. physical symmetry is an analysis of how well and aligned all facet points meet as the exterior of the diamond is analyzed. This is also referred to as meet-point symmetry but there are other features that are examined in the lab such as off centered table, off centered culet, wavy girdle, etc. We have the entire chart of these in our tutorial as well as a high definition video demonstrating these features under a microscope.
Optical symmetry on the other hand is an examination of the optical alignment of the reflections observed within the diamond particularly through optical devices that demonstrate this level of craftsmanship. It's akin to clarity grading through a microscope except we're applying the same principals to cut grading through an H&A viewer."



Lab graded symmetry refers to the physical alignment of the facets of the diamond, the placing of the table facet, culet etc.

http://diamonds.pricescope.com/symm.asp

 

[strmrdr]

Date: 9/18/2009 3:55:02 AM
Author: Lorelei
Here is the cheat sheet some of us use as a guide to find a well cut round diamond.


depth - 60 - 62% - although my personal preference is to allow up to 62.4%

table - 54- 57%

crown angle - 34- 35 degrees

pavilion angle - 40.6- 41 degrees

girdle - avoid extremes, look for thin to slightly thick, thin to medium etc

polish and symmetry - very good and above




note - with crown and pavilion angles at the shallower ends ( CA 34- PA 40.6) and steeper ( CA 35- PA 41) check to make sure these angles complement in that particular diamond - eyeballs, Idealscope, trusted vendor input - check as appropriate!

As the above implies, configurations depend on each other. A little give here can still work with a little take there.

From expert John Pollard.

With that said, here's a 'Cliff's Notes' for staying near Tolkowsky/ideal angles with GIA reports (their numbers are rounded): A crown angle of 34.0, 34.5 or 35.0 is usually safe with a 40.8 pavilion angle. If pavilion angle = 40.6 lean toward a 34.5-35.0 crown. If pavilion angle = 41 lean toward a 34.0-34.5 crown.

GIA 'EX' in cut is great at its heart, but it ranges a bit wider than some people prefer, particularly in deep combinations (pavilion > 41 with crown > 35).

While that list is often repeated it is also somewhat wrong.
That is a cheat sheet for staying near modern Tolkowsky not for finding a well cut diamond.
There are other rules of thumb for 60/60s and chunkies
The maximum pavilion angle for performance is not determined by the pavilion mains angle but the lower half angle for any given crown angle and table size.
http://journal.pricescope.com/Articles/68/1/Do-the-pavilion-mains-drive-light-return-in-the-modern-round-brilliant.aspx

40.5/36/55t/80 lowers is just as valid c/p combo as 40.6/35/55t/80 lowers and can get ags0.
As matter of fact ags0 extends to 40.4/36.7/55/80 lowers.
41/35/55-56t/longer lowers can and often does get ags0
The problematic gia EX combos are the 35.2/41.2 range that are fairly common.

stars - 45-65 is good depending on the crown angle and the upper girdle angle and the pavilion angle.

 

[haagen_dazs]

this is fantastic !
thank you so much!