Question on HCA

[endlessfin1te]

Hey guys,

I thought the HCA may potentially reject good stones, but does it also give good scores to bad stones?


So I got my ASET and i-scope equipments in today to analyze a diamond I bought a few days ago. It had an HCA score of 0.6 so I was mainly hoping to confirm a good purchase.

I don't know how to take pictures of the images...but both the ASET and I-scope images had white leakages resembling "good" in the reference chart. The "ring" center is also pale pink and borderline white.

http://www.ideal-scope.com/1.using_reference_chart.asp

I'm afraid I have a lemon =.= Can HCA misjudge that badly?

 

[JulieN]

Please post the pictures you received from the vendor

 

[Gypsy]

The entire purpose of faceting a diamond is to reflect light.
How well or how poorly a diamond does this determines how beautiful it is.
How well a diamond performs is determined by the angles and cutting. This is why we say cut is king.
No other factor: not color, not clarity has as much of an impact on the appearance of a diamond as its cut. An ideal H will out white a poorly cut F. And GIA Ex is not enough.
So how to we ensure that we have the right angles and cutting to get the light performance we want?
https://www.pricescope.com/wiki/diamonds/diamond-cut
Well one method is to start with a GIA Ex, and then apply the HCA to it. YOU DO NOT USE HCA for AGS0 stones.
https://www.pricescope.com/wiki/diamonds/holloway-cut-advisor
The HCA is a rejection tool. Not a selection tool. It uses 4 data points to make a rudimentary call on how the diamond may perform. It's only goal is to weed out stones that have angles that do not normally result in ideal light performance.
If the diamond passes then you know that you are in the right zone in terms of angles for light performance. Under 2 is a pass. Under 2.5-2.1 is a maybe. 2.6 and over is a no. No score 2 and under is better than any other.
Is that enough? Not really.
So what you need is a way to check actual light performance of your actual stone.
That's what an idealscope image does. https://www.pricescope.com/wiki/diamonds/firescope-idealscope
It shows you how and wear your diamond is reflecting light, how well it is going at it, and where you are losing light return. That is why you won't see us recommending Blue Nile, as they do not provide idealscope images for their diamonds. James Allen, BGD, GOG, ERD, HPD and WF do.

The Idealscope is the 'selection tool'. Not the HCA. The HCA is just used narrow and predict which stones will have a good idealscope image.
So yes, with a GIA stone you need the idealscope images. Or you can buy an idealscope yourself and take it in to the jeweler you are working with to check the stones yourself. Or if you have a good return policy (full refund minimum 7 days) then you can buy the idealscope, buy the stone, and do it at home.


Now if you want to skip all that... stick to AGS0 stones and then all you have to do is pick color and clarity and you know you have a great performing diamond. Because AGS has already done the checking for you. That's why they trade at a premium.

 

[Texas Leaguer]

Light performance is determined by all of the facets working in concert. The proper proportions in combination with three dimensional facet alignment. The HCA tool is looking at only the average of the pavilion and crown angles, along with table and depth. It does not account for the actual measurements of the upper and lower girdle facets or stars, and cannot tell anything about optical precision.

So, yes is it quite possible to have light performance problems even if the HCA score is under 2.

Think of it as physical screen. Larger pieces (over 2) that are likely not to be gems will not pass thru ("rejection"). Smaller pieces that pass through could be gems. Those are the ones to focus (and analyze with additional tools) if you are looking for top cut quality.

As you point out though, there are some very nice stones that score over 2, including some AGS Ideals that have been vetted by a far more sophisticated tool that does look at all the facets in three dimensions.

 

[Gypsy]

Light performance is determined by all of the facets working in concert. The proper proportions in combination with three dimensional facet alignment. The HCA tool is looking at only the average of the pavilion and crown angles, along with table and depth. It does not account for the actual measurements of the upper and lower girdle facets or stars, and cannot tell anything about optical precision.

So, yes is it quite possible to have light performance problems even if the HCA score is under 2.

Think of it as physical screen. Larger pieces (over 2) that are likely not to be gems will not pass thru ("rejection"). Smaller pieces that pass through could be gems. Those are the ones to focus (and analyze with additional tools) if you are looking for top cut quality.

As you point out though, there are some very nice stones that score over 2, including some AGS Ideals that have been vetted by a far more sophisticated tool that does look at all the facets in three dimensions.

Put another way:

HCA usage warnings here: https://www.pricescope.com/wiki/diamonds/holloway-cut-advisor

In Context

Just imagine that you're trying to get to know someone's looks and personality...

An HCA score is merely like having a chalk outline of the person.
Grading report numbers are like having the person's height, weight and clothing measurements.
An ASET or Ideal-Scope (for RB) image is like having a still photo of the person.
An AGS Platinum "0" in performance is like a panel of judges confirming that the person's personality and looks are solid.
A 3D scan in sophisticated cut-calculation software is like having a video interview with the person.

Eventually, it's a lot of great information. All told it's enough for an experienced cut-specialist to make very detailed performance predictions. But in the end, a live date (dinner & a movie?) will be how you finally judge total personality and looks as you, individually, perceive them.

I hope this is helpful.