Calculating pavillion % and crown %

[lady*jane]

Since reading this forum I''ve become much more informed consumer so I''ve been asking for the crown and pavillon angles when out looking at diamonds.

A few folks have told me that I can calcute these angles from the depth % and table %. Is this true? I was going to dusk off the old geometry book but I figured if there was a formula this is the place to ask.

Thanks,
-Jane

 

[Rank Amateur]

Welcome jane!

Because diamonds are three-dimensional multi-faceted objects, the % numbers can't be translated directly into degree angles. I believe that this is why the HCA says that angles are the preferred (more accurate) input. If the angles don't come from a Sarin or other device, then they are no more accurate than the %. Worse yet, if all you have is total depth % then you really don't have much at all to go on.

 

[oldminer]

In theory the accuracy of calculating the angles from percentage and mm measurements is totally precise. In reality, the Sarin and OGI, don't measure the percentages exactly as well as they measure the angles. They do not calculate one from the other as an engineer might do, but they just measure and kind of "round off" in lay terms.

If you have the table and the diameter along with the percentage crown height one can calculate the crown angle with a trig calculator. The same with the pavilion angle from pavilion depth although culet size needs to be either neglected, or measured. This might lead to a bit of innaccuracy.

Garry decided to use angles as he feels they are better measured. I used percentages with AGA as at the time I was developing my charts, percentages were more common discussed.

 

[gemexplorer]

This is my first time in here and I do salute the folks in here where now this is one of the open forum where information to the trade as well to the consumer will benefit.

I welcome the many information given by Holloway and the many system now available to advance the knowledge of diamonds.

This is great for all consumers and for all jewelers.

Information is the most crucial elements between any consumers and the seller and this forum had just the perfect "platform" to make the industry more transparent and this is terrific.

Paul

 

[lady*jane]

Should all diamonds have a Sarin report? Or just the ones that are AGS certified?

The sales people look at me funny when I ask them for the angles and I tell them this information can be found on a Sarin report.

-Jane

 

[Garry H (Cut Nut)]

The original question i think was is it possible to get crown and pav angles from depth and table % data.

Answer = no.

But from the dimensions and these proportions of any diamond from any report you can model a diamond with Diamcalc and get a reliable idea of if it is possible to have a good stone with those proportions. Often you will find that the stone is just impossible to get a good set of propotions that make a nice ideal-scope image (as those often posted by Rich Sherwood and Leonid).

DiamCalc2 costs $280 and can be downloaded from this site, from ideal-scope.com or from cutstudy.com.


re the other comments about accuracy of crown and pavilion angles - using slightly wrong pavilion data in HCA gets very different results. Because most stones have a tiny tiny culet that is too small to be measured, calculating the pav angle from the depth % usually results in a shallower pavilion by about 0.15 degrees. This typically reduces by about 0.5 the accurate HCA score result.

So use angles from Sarin please folks (Sarin measures the angle direct, where as Ogi megascopes calculate the angle).

 

[monkey]

hi
i've been frequenting the forum alot lately too. . .and recently asked the same question. Does somebody have the formulas in order to figure out the crown and pavillion angles. i tried to figure it out myself, but i haven't used trig since highschool.
any further input would be greatly appreciated
thanks