New Princess Crown on Gem Appraisers!?!?

[moremoremore]

Hey Dave....I just noticed the change...when did you make it...Interesting how the 1A crown is 10-15! Was what it before...9 - ?

Was good polish and symmetry always included in the 1A or is that a new change also?

Thank you for providing us with that information!!!

 

[oldminer]

Polish & symmetry have never been active criteria in grading except in unusual circumstances. It would be a rare finely cut diamond with less than very good polish or symmetry, but many or most "good" pol /sym stones still can meet the criteria for an overall grading of fine quality if all else is done properly.

While polish and symmetry have some degree of meaning, they are variables that seem to have only slight effect, in most cases, in setting the overall grade of cut craftmanship.

The crown height in princess cuts has moved up as you noted.

 

[moremoremore]

thanks dave!

 

[SFDavey]

Q: For a princess cut, is crown height measured from the top of the table to the bottom adge of the girdle or the top edge of the girdle?

The diamond I bought my gal seems to have a thick girdle. It is uniform though. As far as I can tell and having compared it (set) visually with the diamonds of about 6 or 7 other gals, it has great sparkle.

The one most important quality of the stone has been met, she loves it, so no worries there.

 

[oldminer]

Crown height is measured in such a stone from the top of the table to the top edge of the girdle. Girdle thickness does not measure into crown height at all.

 

[SFDavey]

Thanks.

Another one for ya...

Q: How does girdle thickness play into the return of light in a princess cut? Certainly the girdle thickness is contributory to the geometry that influences light return...right?

 

[SFDavey]

Shameless ***bump*** in the hopes that this question can be answered.

I get the impression that with cuts like the Princess and Radiant and Asscher and such with larger tables that girdle thickness actually plays a role similar to crown height.

Is there any truth to this idea? Should I start developing a 3-D geometric model to try and find out?

 

[Rhino]

While girdles too thin or too thick are not good (with relation to durability or spread) the girdle has no bearing on light return/leakage. It is the crown/pavilion angle relationship that impacts light return most.

 

[SFDavey]

----------------
On 7/15/2004 12:23:51 PM Rhino wrote:

While girdles too thin or too thick are not good (with relation to durability or spread) the girdle has no bearing on light return/leakage. It is the crown/pavilion angle relationship that impacts light return most.----------------

I have researched and understand why a very thin gridle is bad and why large variation in a girdle is bad.

Most of what I have read regarding crown and pavilion angles is for round stones...not the princess cut or it's derivatives.

When you think of the light entering through the table as it most certainly and predominantly does with a princess cut (or radiant or emerald, etc), it comes in at an angle. The light gets bent upon entry as a function of the physical characteristics of the material (just like water) and continues to travel through the medium until it hits the next 'barrier'. When it reaches the next barrier, depending on the angle of incidence, some is reflected, some again passes through. (There is conceivable an argument for having a lower degree of polish on the lower facets of a diamond to capture the light inside, but then the relection out will be diminished...) By having variations in the depth at which the light (that enters the stone) strikes the bottom facets (from inside) you have variations in the light return.

Heck, I'll go out on a limb here...I don't think that the crown angle has much to do with the return of light from the interior of the stone at all beyond maximizing the area that light has to enter the stone, COUPLED with it's contribution to the depth that light can travel to the lower facets and return back as reflected light. The angle of the lower facets relative to the table strikes me as being SUPREMELY important.

The crown angle definitely has alot to do with sparkles in the sense of having facets that directly reflect light off the surface of the stone. That's a no-brainer. But for light return from the interior of the stone, the distance the light travels and the angle of incidence on the lower facets absoultely controls the amount of light returned from inside the stone.

 

[moremoremore]

OHHH MMAAANNN, I'm gonna stick around for this one!

 

[SFDavey]

I did a quickee search and came up with this stuff for a basic understanding from a physics point of view...

Basics:

http://acept.la.asu.edu/PiN/mod/light/reflection/pattLight1.html

Better:

http://physics.bu.edu/~duffy/PY106/Reflection.html

Diamond specific:

http://www.phy.ntnu.edu.tw/java/propagation/propagation.html

THE classic treatise:

http://folds.net/diamond_design/

Cool visual tool for angles…

http://riker.ps.missouri.edu/rickspage/refract/refraction.html