Ask The Experts All about the HCA ( Holloway Cut Advisor)

[psadmin]

This is the first topic in the Ask the Experts planned series. Our goal here is to use this thread as the ultimate place for the topic. To keep it in one place. We will also create a quick FAQ place from this place.

More on this here:
https://www.pricescope.com/community/threads/faq-and-ask-the-experts.240660/

Please post any question you may have or you think the average lurker or newbie might have. Feel free to answer as well. Garry Holloway the inventor of the HCA will be around to answer questions.

 

[Karl_K]

The HCA is a rejection tool not a selection tool.

I like to think of it as a filter.

It can be used to filter a large list of diamonds down to a smaller list for further consideration.

A good HCA under 2 score does not mean a diamond is well cut.

It means it has the potential to be well cut.


How does it do this?

The HCA looks at the averaged and rounded (grossly rounded in the case of GIA) crown and pavilion angles and tells you if they have the potential to work well together with that table size and other basic measurements.

There are some sanity checks built in to check for potential fish eye and overly thin girdles.



What are some issues with the HCA?

It does not favor diamonds just for rings like some other systems.

It gives a passing grade of under 1 to some combinations that are more suited to pendants and earrings.

I consider this a strength rather than issue, but it does make interpretation more complex.

Another issue is that the HCA is overly harsh on diamonds with pavilion angles of 41 or just over.

They can score up to around 2.5+ and potentially be as good as anything scoring under 2.

On the other hand some combinations score in the same range that the angles are not as well matched.

This creates a bit of a grey zone.

 

[OoohShiny]

It would be useful to include the diagrams showing why lower HCA scores (<1) are not recommended for rings - I think Garry has posted them before somewhere, but I can't remember where!

 

[Garry H (Cut Nut)]

 

[Garry H (Cut Nut)]

Karl did a great job for me
Ohsoshiny does that demonstrate an issue with many shallower stones?

One of the biggest differences between HCA and all other systems is HCA gives a bonus to stones with big spreads. (Using trigonometry based on the 4 proportions you enter we can fairly accurately compute the relative size of a diamond).
What does this mean and why is it important?
Other systems like GIA and AGS give a penalty if the spread is too small. HCA gives an increasing spread point based penalty as spread gets smaller. So the best spread stone gets Zero penalty for a diamond with a 40° pavilion, 32.5° crown, 58% table and a thin girdle. Tolkowsky with a medium girdle comes in at about 0.5.
Why that base? Well it was nearly 20 years ago, but it was a proportion set with a very large 'look' as well as a very large spread.
I have done some recent work and I can tell you, this is infact as large or close to as large as a stone can appear visually.
So hence - as an earring or pendant stone, it will way way look better than a Tolkowsky.

I did an experiment with Drena at one Vegas JCK trip. I put a shallow and Tolkowsky stone into earrings - and asked people to comment and for their preferences. Each day I cleaned them and swapped ears. Even Peter Yantzer (Director of the AGS lab at the time) and almost everyone else chose the shallow stone repeatedly.

 

[Dancing Fire]

I did an experiment with Drena at one Vegas JCK trip. I put a shallow and Tolkowsky stone into earrings - and asked people to comment and for their preferences. Each day I cleaned them and swapped ears. Even Peter Yantzer (Director of the AGS lab at the time) and almost everyone else chose the shallow stone repeatedly.

Garry
Do you think that would be the same case for pendants?

 

[Garry H (Cut Nut)]

Garry
Do you think that would be the same case for pendants?

Definitely

 

[Dancing Fire]

Definitely

When you say shallow, how shallow would you go?

 

[Garry H (Cut Nut)]

When you say shallow, how shallow would you go?

when you go too shallow HCA goes above 2

 

[OoohShiny]

Thanks for the excellent information, Garry!

 

[Paul-Antwerp]

I did an experiment with Drena at one Vegas JCK trip. I put a shallow and Tolkowsky stone into earrings - and asked people to comment and for their preferences. Each day I cleaned them and swapped ears. Even Peter Yantzer (Director of the AGS lab at the time) and almost everyone else chose the shallow stone repeatedly.

I am sorry, Garry, but since the HCA is a rejection-tool, which often needs to be repeated, how can a one-time-test of two stones, without study of how they differ aside from the averages mentioned, be used as proof of a theory?

I remember your experiment very well. You described one, not as a Tolkowsky, but as a H&A, which led me to the question on what theoretical basis you declared one H&A. In case you have forgotten, I declined to cooperate in your experiment, fearing the improper usage of the results.

In my view, you using that experiment in this post is an example of such improper usage.

Live long,

 

[Garry H (Cut Nut)]

I am sorry, Garry, but since the HCA is a rejection-tool, which often needs to be repeated, how can a one-time-test of two stones, without study of how they differ aside from the averages mentioned, be used as proof of a theory?

I remember your experiment very well. You described one, not as a Tolkowsky, but as a H&A, which led me to the question on what theoretical basis you declared one H&A. In case you have forgotten, I declined to cooperate in your experiment, fearing the improper usage of the results.

In my view, you using that experiment in this post is an example of such improper usage.

Live long,

You were afraid to get a result that disagreed with your opinion Paul. We all do that.
I wrote the experiment up in full and posted it with stone proportions etc after the vegas show. You are free to search for it.

 

[bmfang]

Now i’m Curious to read your paper @Garry H (Cut Nut)

 

[blueMA]

You were afraid to get a result that disagreed with your opinion Paul. We all do that.
I wrote the experiment up in full and posted it with stone proportions etc after the vegas show. You are free to search for it.

Garry, could you please share the complete proportions of the diamonds used during the poll at the Vegas JCK trip? I tried searching for the described paper and went nowhere, including both of your sites.

Also, it may help to describe the lighting environment at the facility - I assume people preferred the diamonds optimized for brightness?

 

[Garry H (Cut Nut)]

It is possible I never published the info - I had a quick look and could not find it - I set the stones and did scans etc April 2006 and the JCK Vegas was June 2006 if that helps any sleuths?

I scanned both on Helium and here are the scan info.
Note the highlit bits - AGS 4 and potentially AGS 0.
Table sizes 59.5 shallow and 55.3 H&A. The shallow stone has bad symmetry.
I believe if the 2 diamonds were the same diameter the shallow stone would appear slightly larger in most lighting as it has more light return at the edges.

 

[blueMA]

How interesting - that shallow cut diamond wasn't even a BIC, but does score less than 2 under HCA while penalized under AGS proportions as merely good.

I did notice that diamonds with shallower crowns tend to look bigger and bright especially from a bit of distance with the light return at edges as you describe.

Do you think people may have simply preferred the brightness of the shallow diamond under the given lighting, because the H&A of the given proportions tend to look pretty dark unless in a well-lit environment.

 

[Karl_K]

While Garry's study is not a proof, merely an indication he might be on to something there is a scientific reason it might be true.
The shallower the pavilion the wider angle the stone draws light from around it.
This can be an advantage in earrings where there is a lot of off axis light available.
This is a reason that pear shaped diamonds do so well in earrings.

 

[Karl_K]

I changed blue in ASET to yellow because in an earring obstruction is not strong, I eliminated the red leaving only green to show off axis light draw.

 

[blueMA]

I changed blue in ASET to yellow because in an earring obstruction is not strong, I eliminated the red leaving only green to show off axis light draw.

Karl_K - I just love it when you do your techie things

 

[blueMA]

On an additional note though...my ears get somewhat covered up by long dark hair so unless I have my hair out of the way, the earrings would get funneled ambient light, and actually less axis light available.

I do wonder why the experts tend to assume there's more head obstruction on rings when it's unlikely people will look straight down on the stone to block the light path. Most of people who admire my H&A ring notice it from across the room, not straight down.

 

[Karl_K]

On an additional note though...my ears get somewhat covered up by long dark hair so unless I have my hair out of the way, the earrings would get funneled ambient light, and actually less axis light available.

I do wonder why the experts tend to assume there's more head obstruction on rings when it's unlikely people will look straight down on the stone to block the light path. Most of people who admire my H&A ring notice it from across the room, not straight down.

In the real world obstruction is highly variable and not a neat little circle.
That is a big weakness of all reflector based systems.
When you look down at your ring there is obstruction, when you hold your hand out for others to look at they obstruct the ring.
Across the room takes obstruction out of play.

 

[blueMA]

That is a big weakness of all reflector based systems.
Across the room takes obstruction out of play.

That explains it.

The thing is, when I hold out my hand for an admirer to take a closer look, it's almost always around 45 degrees angle from their heads, not directly underneath where they cast enough shadows unless they pull out a loupe or magnifying glass (has happened!).

People always comment on the laser fire and scintillation of the stone that caught their eyes, because lesser cut diamonds tend to look quite flat in comparison. If this is the effect you're aiming for, then you'd still be better off with ideal cut stones with high optical symmetry with maximal sparkle effects from virtual facets, even in earrings?

 

[Karl_K]

People always comment on the laser fire and scintillation of the stone that caught their eyes, because lesser cut diamonds tend to look quite flat in comparison. If this is the effect you're aiming for, then you'd still be better off with ideal cut stones with high optical symmetry with maximal sparkle effects from virtual facets, even in earrings?

No one is saying that a modern super-ideal wont work well in an earring, they clearly will.
What Garry and I are saying is that they may not be the last word in earring performance.

The vast majority of badly cut stones are steep deep, not shallow by several orders of magnitude.
Cutting for weight retention from the rough leads to steep deep diamonds.
The only time a shallow stone will be cut is if there is no other choice based on the rough.

 

[Karl_K]

That explains it.

The thing is, when I hold out my hand for an admirer to take a closer look, it's almost always around 45 degrees angle from their heads, not directly underneath where they cast enough shadows unless they pull out a loupe or magnifying glass (has happened!).

Even at 45 degrees they are blocking some light, just to a different degree than the reflector model.
Remember that the diamond is seeing the reverse of what you see.
Put yourself in the diamonds place looking back to the viewer and that is what the diamond has to work with.

 

[blueMA]

Even at 45 degrees they are blocking some light, just to a different degree than the reflector model.
Remember that the diamond is seeing the reverse of what you see.
Put yourself in the diamonds place looking back to the viewer and that is what the diamond has to work with.

Perhaps, only if you're situated in certain angle from the available lighting. I just tried moving my ring towards me at 45 angle, and had virtually no effect on the stone, but I'm sitting in a peripheral of a room.

For smaller pendant and earring sizes, I do tend to prefer shallow stones for their brightness, but that often comes at the sacrifice of noticeable laser fire.

 

[Karl_K]

Perhaps, only if you're situated in certain angle from the available lighting. I just tried moving my ring towards me at 45 angle, and had virtually no effect on the stone, but I'm sitting in a peripheral of a room.

ah but you would have to compare it to you not being there, which is impossible.
Distance is what makes the biggest difference.

 

[Karl_K]

For smaller pendant and earring sizes, I do tend to prefer shallow stones for their brightness, but that often comes at the sacrifice of noticeable laser fire.

In lighting highly conductive to fire both can have awesome fire.
In lighting that's between the edge of fire and brightness there could be a point where one is showing brightness and the other fire.
It comes down to lighting.

 

[Garry H (Cut Nut)]

The main reason the trade like deeper tolkowsky than shallow stones is when consumers buy often the biggest most expensive diamond in their life is they are late 20s and have great close up vision.
Most actual "looking and pleasure" of a diamond engagement ring does not come from close looking.
Only the buying.
That is the reason

 

[blueMA]

In lighting highly conductive to fire both can have awesome fire.
In lighting that's between the edge of fire and brightness there could be a point where one is showing brightness and the other fire.
It comes down to lighting.

It's true, many of my play H&A CZs and moissanites also look awesome under certain lighting, but I can easily spot them in more common lighting environment. I covet laser beam off diamonds and sharp on/off scintillation of well-cut diamonds, so I tend to aim for ideal cut rocks.

 

[Garry H (Cut Nut)]

In the real world obstruction is highly variable and not a neat little circle.
That is a big weakness of all reflector based systems.
When you look down at your ring there is obstruction, when you hold your hand out for others to look at they obstruct the ring.
Across the room takes obstruction out of play.