cut advisor question

[evofan]

New to this, sorry. I''ve noticed that when plugging in stats for the HCA, I get better results using pavillion & crown percentages versus angles. Why is that?

 

[valeria101]

Hm...

It just happens. There is always some difference (usually around 1 unit, but it can be quite large indeed) between the HCA scores obtained by angles and by percentages.

The angles are in fact measured directly, averaged, and then percentages get calculated with some approximation: hence the difference and the recommendation to use angles whenever possible for better precission. Also, the indication to use the hCA to select stone with scores above 2 9and not tell apart stones with close scores in the 0-2 interval) should eliminate such measurement bias from your decission).

I cannot know what happened in the case of the stones you tested, however, the scores obtained using percentages can also be 'worse'. On AGS certs, only the averages are available for the angles, so it is not easy to guess what the effect of averaging out was. I suspect that the pavilion angle is a bit understated by the approximative percentage (common occurance), but who knows?

 

[Garry H (Cut Nut)]

Just use %'s if that is all you have - then if the stones look like keepers, ask for the angles.
If you have both only use angles

 

[Lynn B]

I have a question about the HCA, too... I'm never quite sure how to fill in the "Culet" section.

Can someone advise me, please? Many thanks!

Lynn

 

[valeria101]

Hm... you only need it if you use percenatges, not angles. otherwise, "pointed" or "none" for culet means "don't bother" or 0%! The other options are listed in the respective table (red below). As you can see, for each type of culet on the cert, a range of percenatge measures is listed - it should make a really tiny difference if you use max, min or their average - hopefully!

On Sarin reports you get a percenatge for the cuet which can be plugged into the HCA, but then, on Sarin you have those angles so there is no need to use percentages in the HCA (and with angles givem the culet percenatge does not affect the result - you only get a note if it is darn big - but that you will see on the cert anyway and it is not common for modern cuts)

 

[Magnum]

If you use crown/pavilion angles, it's not necessary. It's necessary if you only have crown/pavilion percentages, like what you'd get off an EGL report, because the HCA software needs the culet % to more accurately try to figure out the pavilion angle, which is what it uses, along with the crown angle, depth and table to give you a score.

Edited to add: Oops. I guess Valeria posted while i was typing.

nice pictures, too.

 

[Lynn B]

AnA and Magnum,

THANK YOU so much for the clarification! I truly appreciate your time and patience with me!

Lynn

 

[Garry H (Cut Nut)]

Not quite so simple folks


HCA will also downgrade the score if the culet is too large.


This is for a 9% culet.




Selected: 58% depth, 57% table, 34° crown angle, 40.5° pavilion angle
The result is for a symmetrical diamond with a medium girdle and very good polish
HCA scores were adjusted Dec. 15, 2001 and Feb. 6, 2003.
The culet is clearly visible in the center of this diamond. 1.1 - Excellent
within TIC range

 

[Garry H (Cut Nut)]

try it and you will see the 9% culet will add about 1.0 penalty and will change all sorts of other relationships like girdle allowances etc

 

[Magnum]

Garry, thanks for the info. I did some playing around with the HCA and went back to your website and see what you're talking about. Is it safe to say, though, that as long as the diamond's culet is less than or equal to 3%, or small -> none on a GIA report, there won't be any difference. Or does, say, a 1% culet still affect the score. I didn't see any difference when I plugged my diamond's numbers in and used a culet of 3% or less. Thanks again for the help.

 

[Garry H (Cut Nut)]

yeah, something like that

But remeber that the limitataions of this sort of system is that it works off proportions alone and is not related to size. A 3% culet on a 50ct stone would look shocking, and a 10% culaet on a 0.10ct is hard to see.




One of the things GIA has got right is the method of girdle thickness - it depends more on mm's than %'s

 

[Lynn B]

YIKES! Just when I start to think I'm BEGINNING to figure something out...

Seriously, THANK YOU all for the great input!

Lynn

 

[evofan]

Thanks for the clarifation, everyone. I normally just go with the angle, but curiosity got to me, so I tried the percentages instead. I used it on two diamonds and the results were better with the %.

evofan