Confused by HCA

[ked123]

I input the specs of my diamond in the HCA calculator. The stone scores a 2.8 if I input "crown angle" and "pavilion angle", but if I use the option for "crown %" and "pavilion %" it scores a 1.9.

Why would there be such a discrepancy?

For what it is worth, the description of the cut on the certificate is "excellent".

 

[JulieN]

The angles and percentages are rounded off differently, so you will get different HCA scores. If you got very precise angle and percent measurements, the HCA score will be the same.

 

[Dreamer_D]

Angles are better than the %, because if I am not mistaken, the HCA extrapolates angles from the % to get a score, and that extrapolation process introduces error.

I would trust the angle results more.

But if this stone is a GIA Ex then it could be worth further consideration since GIA averages its angles and that also introduces error. A little error one way or the other could affect the HCA score. If you have seen this diamond and like it and have compared it to others that do scoe under 2, or you are able to get an ideal scope images, that information could trump the HCA.

 

[Garry H (Cut Nut)]

Angles are better than the %, because if I am not mistaken, the HCA extrapolates angles from the % to get a score, and that extrapolation process introduces error.

I would trust the angle results more.

But if this stone is a GIA Ex then it could be worth further consideration since GIA averages its angles and that also introduces error. A little error one way or the other could affect the HCA score. If you have seen this diamond and like it and have compared it to others that do scoe under 2, or you are able to get an ideal scope images, that information could trump the HCA.

Angles have less rounding applied - %'s have far to much rounding!

 

[tweep0]

The answer to your question is on the bottom of HCA page on pricescope... definitely worth reading the entire page if you plan on giving the HCA any value. I've included it below for your convenience:

Angles vs. Percent for Crown and Pavilion

It is best to input crown and pavilion angles into HCA to reject less beautiful diamonds. We use trigonometry to calculate the angles from % data. Use angles if you have them because:

1. %'s are often rounded i.e. 13.3% crown height becomes 13% or 13.5%.
2. Pavilion depth % when converted to pavilion angle often understates the pavilion angle by 0.15 deg
3. Scanners are not good at measuring culets; a culet reduces the pavilion depth % but does not change the pavilion angle.
4. Sarin scanner data is better than Ogi because they calculate the angles and then scale of the %'s. Ogi works the other way round.


If you only have access to % info (e.g. EGL, IGI or HRD report) then use the %'s to reject known duds, then and ask the supplier to run Sarin angle data on your short list.