HCA Confusion and Questions

[metall]

Hi All,

As a long time lurker I have found a lot of very useful information here and I wanted to learn a bit more about the HCA tool and the preferred PS specs.

I had recently been on the diamond hunt for a nice diamond for mom as some of you may know and in my search I came across a diamond with the following characteristics:

.62ct
color: E
Clarity: SI2
Depth: 61.0
Table: 59
Crown Angle: 36.5
Pavillion Angle: 40.2
No culet
Measurements: 5.44 x 5.51 x 3.34

At first I dismissed this diamond outright until I saw the close up video which showed a very clean diamond, so on a whim i inputted the information into the HCA calculator and it came out as a 1.6, which I imagine means that it would have very good light return. Could I ask someone to explain how the HCA calculates light return? and potentially why PSer's look for a more narrow crown and the specific angles for Crown and Pavillion?

Maybe I'm misunderstanding the difference between good light return and ideal cut-ness? Or am I looking at too small a picture? I would love to learn more if someone would explain I would be so grateful!

 

[ChristineRose]

Hi All,

At first I dismissed this diamond outright until I saw the close up video which showed a very clean diamond, so on a whim i inputted the information into the HCA calculator and it came out as a 1.6, which I imagine means that it would have very good light return. Could I ask someone to explain how the HCA calculates light return? and potentially why PSer's look for a more narrow crown and the specific angles for Crown and Pavillion?

Maybe I'm misunderstanding the difference between good light return and ideal cut-ness? Or am I looking at too small a picture? I would love to learn more if someone would explain I would be so grateful!

The HCA doesn't mean good light return. It just means that the diamond is more likely to have good light return than the average diamond. The purpose of the HCA is to help you make a short list of diamonds to investigate when you are looking at thousands of stones on the internet.

Ideal cut can mean whatever you want it to mean--the important question is who called it ideal, and how did they decide it was ideal? Standard definitions start with the proportions (the HCA, more or less), but the good grading labs take more factors into account than does the HCA. For example, if a minor facet is crooked a stone can have an ideal HCA but be far from ideal.

GIA though doesn't use the term ideal. GIA uses excellent, but excellent is a wider range than what most people would consider to be ideal.

As for the narrow crown, all light return is a trade off. More white light = less colored light. Big flashes of light = fewer little sparkles flickering around.

 

[HDer]

The simple answer to your question is a diamond closer to the 60/60 (depth/table) mark, which your diamond is, will return more white light and less colored light than a diamond with a small table.

 

[gm89uk]

HCA assumes perfect symmetry. Best case scenario, 40.2 pavilion will usually lead to obstruction (darkness on close viewing) so not necessarily great for an ering. Try keeping pavilion 40.5+.

Also this combination 40.2/36.5 is less forgiving for variations in LGF length, table size etc and other factors, while the 'ideal' parameters quoted here are more tolerant and you can get good results with a variety of table sizes and LGF, so you're more likely to hit a performer

 

[gm89uk]

The simple answer to your question is a diamond closer to the 60/60 (depth/table) mark, which your diamond is, will return more white light and less colored light than a diamond with a small table.

Sorry HDR 36.5/40.2 diamonds are not the same animal of 60/60 diamonds that people here discuss regularly. The higher crown actually favours fire, but I don't think this combination works well with a 59 table, which may hinder the fire and scintillation.

 

[HDer]

Sorry HDR 36.5/40.2 diamonds are not the same animal of 60/60 diamonds that people here discuss regularly. The higher crown actually favours fire, but I don't think this combination works well with a 59 table, which may hinder the fire and scintillation.

Interesting. Good to know!

 

[rockysalamander]

I was not sure of your budget, but here are two diamonds that have good angles.

Around $3k, this is a great choice. AGS0 and good angles. https://www.jamesallen.com/loose-di...rat-d-color-vs2-clarity-ideal-cut-sku-3267883

Around $2k. https://www.jamesallen.com/loose-di...e-color-vs1-clarity-excellent-cut-sku-3491398 {need a IS}

 

[metall]

The HCA doesn't mean good light return. It just means that the diamond is more likely to have good light return than the average diamond. The purpose of the HCA is to help you make a short list of diamonds to investigate when you are looking at thousands of stones on the internet.

Ideal cut can mean whatever you want it to mean--the important question is who called it ideal, and how did they decide it was ideal? Standard definitions start with the proportions (the HCA, more or less), but the good grading labs take more factors into account than does the HCA. For example, if a minor facet is crooked a stone can have an ideal HCA but be far from ideal.

GIA though doesn't use the term ideal. GIA uses excellent, but excellent is a wider range than what most people would consider to be ideal.

As for the narrow crown, all light return is a trade off. More white light = less colored light. Big flashes of light = fewer little sparkles flickering around.

Thanks ChristineRose!

This somewhat makes sense to me, but I may just not be well versed enough to understand fully. Please let me know if I've got this right: HCA only calculates potential light return...but the TYPE of light return is dependant on the dimensions?

So sorry my use of ideal is probably very wrong in that sentence...it was a gia graded diamond....i don't recall whether it was triple x...I didn't save it down because it didn't fall in the PS range. I'm just using this diamond as an example for my question.....since I have run into a few no PS specs that have great HCA scores.

As for the last paragraph i sort of want to pick your brain....say if I have a diamond which shines very white but not a lot of rainbows....would that mean it could potentially have a good hca score but the way it is cut causes it not to throw rainbows? What would cause a diamond to shine white instead of throw rainbows? What differentiates big flashes versus little rainbows?

 

[metall]

I was not sure of your budget, but here are two diamonds that have good angles.

Around $3k, this is a great choice. AGS0 and good angles. https://www.jamesallen.com/loose-di...rat-d-color-vs2-clarity-ideal-cut-sku-3267883

Around $2k. https://www.jamesallen.com/loose-di...e-color-vs1-clarity-excellent-cut-sku-3491398 {need a IS}

Thank you rocky! I've actually been working with idj for my diamond....I was just using the specs above for this question, it's not even part of my maybe list...though that second stone you showed....!!!!!

 

[metall]

HCA assumes perfect symmetry. Best case scenario, 40.2 pavilion will usually lead to obstruction (darkness on close viewing) so not necessarily great for an ering. Try keeping pavilion 40.5+.

Also this combination 40.2/36.5 is less forgiving for variations in LGF length, table size etc and other factors, while the 'ideal' parameters quoted here are more tolerant and you can get good results with a variety of table sizes and LGF, so you're more likely to hit a performer

So sorry for this but may I ask what LGF is? I'm a super newbie!

 

[metall]

Sorry HDR 36.5/40.2 diamonds are not the same animal of 60/60 diamonds that people here discuss regularly. The higher crown actually favours fire, but I don't think this combination works well with a 59 table, which may hinder the fire and scintillation.

May I ask what a 60/60 stone is?

Wondering now if there is a PS dictionary where I can look up terms....so sorry for my ignorance.

 

[OoohShiny]

So sorry for this but may I ask what LGF is? I'm a super newbie!

May I ask what a 60/60 stone is?

Wondering now if there is a PS dictionary where I can look up terms....so sorry for my ignorance.

LGF = Lower Girdle Facets

60/60 = 60% table, 60% depth

 

[ChristineRose]

I wouldn't say that the HCA calculates potential light return. I'd say it calculates light return based on only three numbers, when hundreds of numbers are needed to get a complete picture. These three numbers are the most important of course. Even a sort of bad HCA has the potential for good light return, but those hundreds of other numbers have to be chosen very carefully.

Truly bad HCA numbers will always mean bad light return, but even then the diamond can be pretty. A lot of antique stones were cut for candlelight and for facet pattern for example.

 

[Karl_K]

With a 40.2 pavilion you get some color entrapment, in other words it can face up with more color than the grade indicates. This can be lessened a bit with precision cutting and optical alignment but it exists. There will also be obstruction issues in a ring that can be minimized by long lower girdle facets.
Overall its not a combo I would recommend in this case.
Precision cut with a 55% table and a 80% lgf that combo could knock you sock offs but would have slight color retention.
I have seen such a diamond however and it was swoony!
This isn't one of them.

 

[metall]

With a 40.2 pavilion you get some color entrapment, in other words it can face up with more color than the grade indicates. This can be lessened a bit with precision cutting and optical alignment but it exists. There will also be obstruction issues that can be minimized by long lower girdle facets in a ring.
Overall its not a combo I would recommend in this case.

Thank you Karl! This was super helpful. I imagine that I didn't see much of the color entrapment you're speaking about here because it was a e- colored diamond. But it does explain why i felt that overall the stone was a bit dull.

I love how precise the science for the diamond cutting is and can't wait to learn more.

 

[metall]

LGF = Lower Girdle Facets

60/60 = 60% table, 60% depth

Thank you shiny!

Could I ask someone to explain what makes a 60/60 diamond better or worse than other table/depth combos? is this a sought after cut? and for what reasons? What crown and pavilion angles would go with this table/depth combo?

 

[metall]

I wouldn't say that the HCA calculates potential light return. I'd say it calculates light return based on only three numbers, when hundreds of numbers are needed to get a complete picture. These three numbers are the most important of course. Even a sort of bad HCA has the potential for good light return, but those hundreds of other numbers have to be chosen very carefully.

Truly bad HCA numbers will always mean bad light return, but even then the diamond can be pretty. A lot of antique stones were cut for candlelight and for facet pattern for example.

Gotcha! So so so cool.

What would be considered a truly bad HCA #?

 

[OoohShiny]

My understanding is that a bigger table reduces 'fire', because light comes straight back out the top rather than out the star facets, the latter of which (as I understand it) help light dispersion and therefore the creation of the coloured fire that you see. A smaller table means bigger star facets, therefore more opportunity for fire.

60/60 is therefore known as a combination that gives more white light return than coloured fire, although it is personal taste as to what you prefer.

@Karl_K can likely give a better explanation of the above than I can!

 

[gm89uk]

I wouldn't say that the HCA calculates potential light return. I'd say it calculates light return based on only three numbers, when hundreds of numbers are needed to get a complete picture. These three numbers are the most important of course. Even a sort of bad HCA has the potential for good light return, but those hundreds of other numbers have to be chosen very carefully.

So utilising those three numbers, my understanding is that it calculates the potential light return, should everything else be perfect.

No matter how you slice it and dice it, to my understanding, HCA 3 stone (everything else being tiptop) won't have the same volume of light reflected as HCA 1.5. although this is totally different to which is more beautiful, or which is more brilliant.

 

[Karl_K]

My understanding is that a bigger table reduces 'fire', because light comes straight back out the top rather than out the star facets, the latter of which (as I understand it) help light dispersion and therefore the creation of the coloured fire that you see. A smaller table means bigger star facets, therefore more opportunity for fire.

60/60 is therefore known as a combination that gives more white light return than coloured fire, although it is personal taste as to what you prefer.

@Karl_K can likely give a better explanation of the above than I can!

You did really good.
I would add that its not the amount of fire or the intensity of fire that is hugely different in lighting that is highly conductive for fire. In lighting highly conductive to fire 60/60 could have "better" fire depending on a zillion factors.
The biggest difference is in how many different lighting conditions it shows fire.
Well cut Ideal cuts tend to show fire in more lighting conditions than a well cut 60/60.
In the same way a well cut oec shows fire in more lighting conditions than an ideal cut or 60/60.
When you start talking wonky, well then all bets are off.