Help understanding Cut vs. HCA

[Enrico81]

Hi everyone, I am new and still at the bottom of the learning curve, but it seems to me that the amount of light reflected back from a diamond is closely related to both Cut and HCA. Yet, I don't understand the difference between the two and which one is more important when it comes to the brilliance and fire of a diamond.

For example, when I look at this diamond: http://www.eternitydiamonds.com/diamond_detail.php?id=436088

the Cut is deemed Very Good but the HCA is calculated as 1.7. Isn't this contradictory? I thought that anything under an HCA of 2 would mean that it is in the top 5% of performers? So how could this be classified as just a Very Good cut?

Also, according to the GIA report, it is a VS2 with crystals as being the only imperfections worthy of note and when you look at the price/ct you can see that it is low. So how should I interpret this? Or would one have to visually inspect the diamond to make sense of all this?

 

[Gibson486]

HCA is just a weeding out tool.

To get Ex cut, you just need to be with in the Ex. cut spec. This means that it could include a pav. angle and a crown angle that fall within the Ex parameters, but those angles may not play with each other well...HCA helps weed out those diamonds.

i have posted this question in the past. Yes, it is possible to get a VG grade diamond to outshine an Ex grade.

Also, it's the 5% of ALL diamonds, not just Ex. cut ones.

In the end, your eyes are the final judge.

 

[0-0-0]

The diamond is downgraded by GIA to a very good cut due to the good polish, good symmetry, and thick girdle. I'm not sure whether the HCA is valid in this case as it assumes "a symmetrical diamond with a medium girdle and very good polish".

HCA gets no info on symmetry, polish and minor facets; use it only to reject likely bad performing diamonds to narrow down your final selection. Ideal-Scope images and independent appraisers can help after that.

Even in cases with "a symmetrical diamond with a medium girdle and very good polish", it's not contradictory that the HCA disagrees with GIA or AGS. They follow different guidelines after all.

 

[diamondseeker2006]

It is recommended to go with a minimum of very good for polish and symmetry, and girdles should be between thin to slightly thick.

 

[Enrico81]

The diamond is downgraded by GIA to a very good cut due to the good polish, good symmetry, and thick girdle. I'm not sure whether the HCA is valid in this case as it assumes "a symmetrical diamond with a medium girdle and very good polish".

HCA gets no info on symmetry, polish and minor facets; use it only to reject likely bad performing diamonds to narrow down your final selection. Ideal-Scope images and independent appraisers can help after that.

Even in cases with "a symmetrical diamond with a medium girdle and very good polish", it's not contradictory that the HCA disagrees with GIA or AGS. They follow different guidelines after all.

Mmm interesting. I hadn't considered that but it makes sense. I also noticed that the 62.4% depth does not give a great spread for a diamond of comparable weight and proportions. Might this account for the low price/ct?

 

[Enrico81]

It is recommended to go with a minimum of very good for polish and symmetry, and girdles should be between thin to slightly thick.

Thanks for the tip!

 

[0-0-0]

I also noticed that the 62.4% depth does not give a great spread for a diamond of comparable weight and proportions. Might this account for the low price/ct?

I think the good polish and symmetry accounts for the low price/ct in this case. The diameter is on par with other 0.80 ct stones.