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

[blueMA]

Garry, would you elaborate please?

 

[Garry H (Cut Nut)]

Garry, would you elaborate please?

It is a human obstruction ASET I also desiged and tested in 2006.
AGS made the blue too large - they assumed a cyclops, and we each have an eye close to the side of our head. The stripe inside is the human body. I developed and tested it on shallow stones to get an idea about the effects of looking at earrings and pendants.
Thoiught it answered Karl's line of comments rather well

 

[blueMA]

It is a human obstruction ASET I also desiged and tested in 2006.
AGS made the blue too large - they assumed a cyclops, and we each have an eye close to the side of our head. The stripe inside is the human body. I developed and tested it on shallow stones to get an idea about the effects of looking at earrings and pendants.
Thoiught it answered Karl's line of comments rather well

Got it - but in all reality, most people who notice someone else's e-ring wouldn't be standing right over it or be right next to it to cast a shadow over the stone. Even when I lend out my hand for a closer look (which mostly occurred right after the engagement a long ago), I simply reached out my hand from where I was and didn't necessarily get close enough to the viewer to block a light path. This of course would depend on the available ambient lighting, but in majority of situation where I caught people staring at my ring, it was from across the table or a room.

 

[Garry H (Cut Nut)]

You can turn the scope around to see what the other person sees from their light blocking point of view
We are off topic

 

[blueMA]

I do find HCA very useful while searching for diamonds so I'm grateful of the availability of the tool. I often find a diamond of interest get penalized for spread that I tend to accept as long as it's within the AGS ideals, but when a diamond gets penalized for the scintillation it tends to concern me. Would you please explain the science behind determining the scintillation score?

 

[Garry H (Cut Nut)]

I do find HCA very useful while searching for diamonds so I'm grateful of the availability of the tool. I often find a diamond of interest get penalized for spread that I tend to accept as long as it's within the AGS ideals, but when a diamond gets penalized for the scintillation it tends to concern me. Would you please explain the science behind determining the scintillation score?

https://www.diamond-cut.com.au/13_scint.htm is what I wrote 19 years ago, and largely valid today.
In this article there is a lot more and way better info. Back then there was no way to get accurate or easily get lower girdle info, and that has a big effect on scintillation. But you can see from the chart on the Diamond-cut link, that the better scores line up with non leaky and not to shallow stones.
Basically I was looking for start patterns that turn on and off - unlike nail heads or bowtiesthat are persistant.

 

[blueMA]

This explains exactly why the HCA could be overly harsh for diamonds with pavilion angles of 41 and just over. The predictor model assumes increased chance of ring light leakage and thinning star pattern as the lower girdle length increases, but this isn't necessarily so in well cut diamonds with balanced star patterns. This means I can easily rule this out as well with access to ASET and Idealscope images. Thanks!

 

[Garry H (Cut Nut)]

This explains exactly why the HCA could be overly harsh for diamonds with pavilion angles of 41 and just over. The predictor model assumes increased chance of ring light leakage and thinning star pattern as the lower girdle length increases, but this isn't necessarily so in well cut diamonds with balanced star patterns. This means I can easily rule this out as well with access to ASET and Idealscope images. Thanks!

Blah blah blah. Blue Ma 41 is not a limit if you reduce crown angle.

 

[blueMA]

Blah blah blah. Blue Ma 41 is not a limit if you reduce crown angle.

Exactly my point! I’m rather fond of 41 pavilion with appropriate shallower crown.

 

[WinkHPD]

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.

My wife and I are solidly in your camp BlueMA. She wears a pair of CBI earrings that are half a carat each. It is rare that someone does not comment to her, often total strangers, on how incredibly sparkly they are.

Wink

 

[blueMA]

My wife and I are solidly in your camp BlueMA. She wears a pair of CBI earrings that are half a carat each. It is rare that someone does not comment to her, often total strangers, on how incredibly sparkly they are.

Wink

Exactly! Well cut diamonds throw off some far reaching wowza beams.

 

[WinkHPD]

A little off topic, but several years ago, Garry showed me two glass topped containers each with a fairly large number of one or two point diamonds in them. Garry will remember the size and the number of diamonds in them much better than I.

He showed them to me and asked me which I thought was the most beautiful collection. I favored one box and was surprised to learn that it contained only well cut single cuts and the other, contained very well cut full cut round brilliants.

Once he explained it to me, it fit right in with what I tell my clients. The single cuts (seventeen facets instead of 57) allowed for virtual larger facets, which set up larger flashes of both white and colored light. It is the size of these virtual facets, whether in small or large diamonds that determines the beauty to the eye of the observer. In slightly larger diamonds, the 57 facet diamond adds a wonderful dazzling sparkle when properly cut, but in tiny diamonds, they are too small to be fully appreciated by the human eye.

One thing I have learned about Garry Holloway, he always makes me think!

Wink

 

[blueMA]

I wouldn't care how melee are cut as long as they sparkle, and it makes sense bigger facets would throw bigger flashes of light at such tiny sizes. In fact, that's probably why OEC diamonds are coming back into vogue for such effect.

 

[Garry H (Cut Nut)]

A little off topic, but several years ago, Garry showed me two glass topped containers each with a fairly large number of one or two point diamonds in them. Garry will remember the size and the number of diamonds in them much better than I.

He showed them to me and asked me which I thought was the most beautiful collection. I favored one box and was surprised to learn that it contained only well cut single cuts and the other, contained very well cut full cut round brilliants.

Once he explained it to me, it fit right in with what I tell my clients. The single cuts (seventeen facets instead of 57) allowed for virtual larger facets, which set up larger flashes of both white and colored light. It is the size of these virtual facets, whether in small or large diamonds that determines the beauty to the eye of the observer. In slightly larger diamonds, the 57 facet diamond adds a wonderful dazzling sparkle when properly cut, but in tiny diamonds, they are too small to be fully appreciated by the human eye.

One thing I have learned about Garry Holloway, he always makes me think!

Wink

It is a bit OT Wink, but here they are:

is a video of the stones in 3 different lightings.
its all a bit rough - just did it with 2 year old samsung phone. Plastic box has been in my pocket a fair bit and is scratched.
the photo's are in spot and office light - the camer sees 100th of the fire your eyes do.

 

[McGinnis]

that's amazing. Going only single cuts for side stones from now on!

 

[WinkHPD]

Thank you Garry, coincidentally, this matches up with everything John and Paul have taught me about visibility of dispersion.

Wink