HCA does not match GIA cut grade

[oldminer]

Over the years we have seen dozens of threads concerning the fact that the HCA often is at odds with the GIA cut grade methodology. I have come to regard the HCA as an excellent prediction of performance while I have also seen good reason to have some degree of confidence in the GIA Excellent grade. Sometimes the HCA will indicate a low performance range on a EX cut GIA graded diamond and my experience is that the HCA is right yet the diamond is parametrically a very nicely cut stone although its "look" is not quite up to the top end light behavior-wise.
At other times the HCA gives a very high performance prediction to a GIA Very Good or Good stone. Usually this sort of diamond still will look very good to me on a light return basis, but probably I will be able to pick out the physical reasons why it is graded lower by GIA, too.

The two systems were not created by entities which work in consort with eachother. They both arrive at meaningful results most of the time, but often enough there are some disconcerting conflicting results. The HCA gives a very good idea of the quality of appearance while the GIA report often gives a decent summation of quality by parameter although it was intended to grade light return more than it seems to have succeeded in doing.

A GIA EX with a high HCA is a no-brainer. A GIA VG or Good with low HCA is also taken care of. When I see a diamond with a high HCA and conflicting, lower GIA report, I am forced to look at the finer points of the parametric measures provided by the Sarin. When I see a high GIA grade (EX) and a low HCA, I tend to tell people to simply look for another diamond. No point in paying a premium price for a questionable result.

 

[Fly Girl]

Date: 6/26/2008 2:08:31 PM
Author:oldminer
Over the years we have seen dozens of threads concerning the fact that the HCA often is at odds with the GIA cut grade methodology. I have come to regard the HCA as an excellent prediction of performance while I have also seen good reason to have some degree of confidence in the GIA Excellent grade. Sometimes the HCA will indicate a low performance range on a EX cut GIA graded diamond and my experience is that the HCA is right yet the diamond is parametrically a very nicely cut stone although its ''look'' is not quite up to the top end light behavior-wise.
At other times the HCA gives a very high performance prediction to a GIA Very Good or Good stone. Usually this sort of diamond still will look very good to me on a light return basis, but probably I will be able to pick out the physical reasons why it is graded lower by GIA, too.

The two systems were not created by entities which work in consort with eachother. They both arrive at meaningful results most of the time, but often enough there are some disconcerting conflicting results. The HCA gives a very good idea of the quality of appearance while the GIA report often gives a decent summation of quality by parameter although it was intended to grade light return more than it seems to have succeeded in doing.

A GIA EX with a high HCA is a no-brainer. A GIA VG or Good with low HCA is also taken care of. When I see a diamond with a high HCA and conflicting, lower GIA report, I am forced to look at the finer points of the parametric measures provided by the Sarin. When I see a high GIA grade (EX) and a low HCA, I tend to tell people to simply look for another diamond. No point in paying a premium price for a questionable result.

Just checking to see what you mean by "high HCA" and "low HCA". I think what you mean by "high HCA" is a good HCA score which is a number less than 2. Similarly, a "low HCA" is a poor HCA result, which is a high HCA number (above 2).

Good comments, though.

 

[oldminer]

The better the HCA, the lower numerically.

 

[MikeRato1]

Date: 6/26/2008 4:16:01 PM
Author: Fly Girl

Date: 6/26/2008 2:08:31 PM
Author:oldminer
Over the years we have seen dozens of threads concerning the fact that the HCA often is at odds with the GIA cut grade methodology. I have come to regard the HCA as an excellent prediction of performance while I have also seen good reason to have some degree of confidence in the GIA Excellent grade. Sometimes the HCA will indicate a low performance range on a EX cut GIA graded diamond and my experience is that the HCA is right yet the diamond is parametrically a very nicely cut stone although its ''look'' is not quite up to the top end light behavior-wise.
At other times the HCA gives a very high performance prediction to a GIA Very Good or Good stone. Usually this sort of diamond still will look very good to me on a light return basis, but probably I will be able to pick out the physical reasons why it is graded lower by GIA, too.

The two systems were not created by entities which work in consort with eachother. They both arrive at meaningful results most of the time, but often enough there are some disconcerting conflicting results. The HCA gives a very good idea of the quality of appearance while the GIA report often gives a decent summation of quality by parameter although it was intended to grade light return more than it seems to have succeeded in doing.

A GIA EX with a high HCA is a no-brainer. A GIA VG or Good with low HCA is also taken care of. When I see a diamond with a high HCA and conflicting, lower GIA report, I am forced to look at the finer points of the parametric measures provided by the Sarin. When I see a high GIA grade (EX) and a low HCA, I tend to tell people to simply look for another diamond. No point in paying a premium price for a questionable result.

Just checking to see what you mean by ''high HCA'' and ''low HCA''. I think what you mean by ''high HCA'' is a good HCA score which is a number less than 2. Similarly, a ''low HCA'' is a poor HCA result, which is a high HCA number (above 2).

Good comments, though.

what is your first thought when you see a high hca score and gia ex''s, or did you explain that and i am just confused?