Really confused on the HCA score & how to use it

[cypp]

I am really confused on how to use the HCA score to determine which is the best diamond to buy. The description on the page says, Most people prefer stones that rate 1-2 on a scale where: 0-2 Excellent, 2-4 Very Good, 4-6 Good, 6-8 Fair, and 8-10 Poor. Zero is almost impossible since many of the factors conflict.

Does that mean a stone that rates 0.00 - 2.00 is the best buy? Or 1.00 to 2.00?

I posted on another thread about three diamonds we were considering... one stone scored .8 and the other 2.3. The stone that scored 2.3 was said not to be ideal... but the cut, polish, and symm were all excellent and the "X" mark was inside both the GIA and AGS ideal candidates... what made that stone not a good buy?

 

[JulieN]

if you have seen the 2.3 and like it, go for it. the theory is that up to 2.5 can still be attractive when put into open settings.

also, if the angle is actually 41.1 due to GIA rounding, the stone will score under 2.

 

[cypp]

Hello... the pavillion angle is 41.2 for the stone that scored 2.3

The Light return, Fire, Scintillation, and Spread all graded "Very Good" and the score 2.3 but on the stone that scored 0.8 it all came up as "Excellent"

Is the HCA reliable?

It''s really confusing to use... I thought anything that scored below 2 is considered a good diamond?

For a diamond that scores above 2, does that mean it is not a good diamond and should be avoided??

 

[niceice]

The HCA is an excellent tool that is designed to provide a basic insight into the "potential" of a stone, it is merely the beginning of the selection process... It can not be more than that because it merely takes the average measurements of a diamond into consideration and thus it can''t tell you more than the potential visual performance of the stone... I think of it as an excellent way to initially sift through the myriad of possibilities, for instance, if you conducted a search for 1.00 - 1.10 carat, H color, VS-2 clarity ideal cut diamonds, you might get twenty results. Use the HCA to narrow down the list of possibilities and then carefully consider the overall aspects of the top five or so stones, such as:

The diamond has a pavilion angle of 41.2 degrees... What are the other measurements? How do they interact with each other? What are the high and low measurements of the pavilion facets that the average measurement of 41.2 degrees was derived from? To what degree of consistency were the pavilion facets cut in relationship to each other? I won''t go into a lot of detail here, because if you search for HCA here on the forum this subject has been addressed on many other occasions in great detail...

What are the inclusions / flaws / clarity characteristics within the diamond? Where are they located? How do the inclusions within this diamond compare to the inclusions (type, location and visibility) within another diamond with comparable characteristics?

Ask the vendor to provide you with clear clarity photographs of the diamond that provide you with additional detail regarding the inclusions within the diamond.

Some of the vendors here on PS provide additional insight into the light performance of the diamond...

Good luck with your quest, the tutorials here on PS are a great asset so read away!

 

[cypp]

Hi Todd,

Thank you very much for your response.

We have been looking for over two months, and thought we had narrowed down our search to three stones, all triple Excellent, and ideal table and depth. I didn''t know how to use the crown angle/pavillion angle until coming here on Pricescope. I''m just confused each time I use it because .8 was said not to be a good stone even though it scored "Excellent" on the HCA.

Would a stone that scores 2.3 or 2.4 on the HCA be considered as not a good buy?

 

[cypp]

From a response on a previous thread, a score of a 0.8 is not good... ? But when I put in the numbers:

62.3% depth, 55% table, 35.5° crown angle, 40.4° pavilion angle

it says:

Light Return Excellent
Fire Excellent
Scintillation Excellent
Spread
or diameter for weight Very Good
Total Visual Performance 0.8 - Excellent
within TIC range

I looked at another stone yesterday that scored 1.3 but each factors came back as "Very Good" not "Excellent:

How come the less ideal score (. scores better (Excellent) than the ideal score (1.3)?

I hope I make sense... I''m just confused as to how to use this.

 

[whatmeworry]

Cypp,
Relax. You''re starting to overanalyze the diamonds. The 2.3 diamond is worth looking at as is the 0.8. I''ve seen some of the PS vendors'' Heart and Arrow diamonds have HCA scores in that range. It really comes down to your own preference. If you have visually seen them and prefer one, go for it and enjoy it. Good luck.

 

[JulieN]

.8 is not worse than 1.3

 

[Regular Guy]

Date: 7/10/2006 3:43:02 PM
Author:cypp

Does that mean a stone that rates 0.00 - 2.00 is the best buy? Or 1.00 to 2.00?

Cypp,

You probably will not be particularly relieved to know that others have asked the same questions you have.

See this post, but go to page 1. I''ve linked to page 4, in the hope that Paul (and maybe even Garry will take a second look) will review my first post on page 4 here, and see if, even if Garry doesn''t think so, I''m touching on his point with how the HCA is described as working.

 

[Arcam]

"I looked at another stone yesterday that scored 1.3 but each factors came back as "Very Good" not "Excellent:"

the "very good" and "excellent" ratings are imperfect measures in my limited experience. I had two diamonds shipped to an appraiser, one scored a 1.3 on the HCA the other a 1.9. The 1.3 scored an excellent on Light Return, Fire, and Scintillation, while the 1.9 scores excellent on light return, but only very good on fire and scintillation. To my eyes, the diamond with the 1.9 score had much better fire and scintilation than the diamond that scored 1.3 on the HCA. While I think the HCA is a useful tool, your eyes may come up with a different result and are more important than what its HCA score is.

 

[Paul-Antwerp]

Date: 7/12/2006 4:22:31 PM
Author: Regular Guy

Date: 7/10/2006 3:43:02 PM
Author:cypp

Does that mean a stone that rates 0.00 - 2.00 is the best buy? Or 1.00 to 2.00?

Cypp,

You probably will not be particularly relieved to know that others have asked the same questions you have.

See this post, but go to page 1. I''ve linked to page 4, in the hope that Paul (and maybe even Garry will take a second look) will review my first post on page 4 here, and see if, even if Garry doesn''t think so, I''m touching on his point with how the HCA is described as working.

No, no, no. To the contrary.

In Gary''s HCA, the stones scoring closer to 0 are going closer to the edge. That is why, in general circumstances, a lot of people prefer the stones scoring between 1 and 2, since they are safely away from the edge, and since the HCA (only working on averages) cannot make a distinction between those having very tight variances and those that do not.

We have produced a high number of stones scoring very close to 0 (0.4 or thereabouts), and since they have very tight variances, they are the top of the cream. But for every stone that we produce, you can probably find about 100 with less tight variances, and the same score of 0.4, and these will be clearly less performing. The key is in tightness and symmetry, not in the hot-spot being between 1 and 2.

This is very important, so, if I am not totally clear, please ask for clarification.

Live long,

 

[Regular Guy]

So, there are two things here.

a) First, I''m not sure if I understand, really your point here...

Date: 7/12/2006 4:42:12 PM
Author: Paul-Antwerp

In Gary''s HCA, the stones scoring closer to 0 are going closer to the edge. That is why, in general circumstances, a lot of people prefer the stones scoring between 1 and 2, since they are safely away from the edge,

And, you know, Paul, many people have gotten into the habit of knocking stones below 1, and sometimes describing the pattern there as their having less contrast. Whether it''s true or not, I''m curious about the imbedded math that must be involved

b) and then...since HCA in principle is a linear sort of instrument, maybe I should understand the current text...

"Stones near the center of the red region (the lowest scores) are least affected by symmetry variations. Alternatively hearts and arrows diamonds, which have excellent optical symmetry, but often HCA scores around 2, may out-perform diamonds with lesser symmetry and lower HCA scores."

in this sort of way:

1) best are symmetrical stones near zero
2) next best, stones nearer 2, but still with with high optical symmetry
3) not quite as good, but still pretty good...stones nearer 0, but with less optical symmetry

If this is what''s meant, it''s clearer to me.

 

[Garry H (Cut Nut)]

You "guy''s" seem 2 have this all under control.

Good work chap''s. Back to appraisals as our valuer is on vacation

 

[cypp]

Thanks so much everyone for your response and explanation. I''m going to read the previous thread. I guess I am over analyzing it a bit... I''m trying to understand it.

Another quick question?

What is the difference between the BIC range and TIC range? I clicked on the link to it, but couldn''t figure it out...

 

[JulieN]

A BIC is optimized to return white light. A FIC is optimzed to return fire. A TIC is a balance between the two.

 

[Paul-Antwerp]

Date: 7/12/2006 5:05:41 PM
Author: Regular Guy
So, there are two things here.

a) First, I''m not sure if I understand, really your point here...

Date: 7/12/2006 4:42:12 PM
Author: Paul-Antwerp

In Gary''s HCA, the stones scoring closer to 0 are going closer to the edge. That is why, in general circumstances, a lot of people prefer the stones scoring between 1 and 2, since they are safely away from the edge,

And, you know, Paul, many people have gotten into the habit of knocking stones below 1, and sometimes describing the pattern there as their having less contrast. Whether it''s true or not, I''m curious about the imbedded math that must be involved

The thing is the following. In the pavilion, somewhere around 40.5° or lower, the light performance goes down sharply. But closer to that edge, light performance still gets better. Since the HCA also is positive for stones with more spread, all other proportions being equal, the spread with a lower pavilion angle get even a better HCA-score.

Now, in reality, the majority of stones do not have the best symmetry. I am not only talking about H&A, symmetry goes much further than H&A, and H&A is definitely not the ultimate. So, in most stones, the closer the average is to that edge, the more single facets are on the wrong side of the edge, and this will result in a less performing stone.

However, a cutter who takes care of ultimate symmetry, will get a stone with the same averages, and thus the same HCA-score, but not one facet on the wrong side of the edge. Bingo, you will have the ultimate winner.

Unfortunately, of 100 stones with an HCA under 1, only very few will have such ultimate symmetry. So, the average is hiding the top.

Live long,

 

[Regular Guy]

Paul,

Thanks for your comments here, which -- upon rereading them -- do help my understanding.

Of course, they make me want to consider the difference between, and place a value upon, the variance between Superideal and Ideal, but now I''m just recycling myself.

In contrast, I think your comments help bring a genuine understanding to the real linearity of the HCA, which...as you say...can get lost in the execution, causing the 1 - 2 options to be the relatively safer target area.

Interesting stuff.

 

[WinkHPD]

Date: 7/13/2006 5:49:40 PM
Author: Regular Guy

In contrast, I think your comments help bring a genuine understanding to the real linearity of the HCA, which...as you say...can get lost in the execution, causing the 1 - 2 options to be the relatively safer target area.

Interesting stuff.

This has been an area of dissapointment for me as I see stones that have lower than 1 hammered as not being nice when I have them in front of me and know them to be extraordinary. I often wondered what people were smoking to say such horrible things.

Thank you Paul, for helping me also to understand this phenomina better. It is amazing how much you can know, and still know nothing...

Wink

 

[Peerless]

Paul,

When you talk about "tight variances", what are you referring to? Pavilion angles in relation to each other and corresponding crown angles? Can you please clarify what is most important in looking for a "tight" stone?

Thanks,
Peerless

 

[Paul-Antwerp]

Date: 7/13/2006 7:27:57 PM
Author: Peerless
Paul,

When you talk about ''tight variances'', what are you referring to? Pavilion angles in relation to each other and corresponding crown angles? Can you please clarify what is most important in looking for a ''tight'' stone?

Thanks,
Peerless

Tight variances means roundness, or squareness in square stones. The same or close to the same main pavilion angle, then the same or close to the same angle on the pavilion halves, resulting in the same length of lower-girdle-facets.

Similar in the crown area.

And of course, all facets nicely opposite to eachother.

And always, the pavilion is more important than the crown.

Hope that this is clear,

 

[Sunshines]

Hi Paul,

Sorry, but I still cant figure out the definition of tight. If I give you the real specs of a stone that scores 0.4 HCA, can you tell me what are the measurements or items here that will indicate if its tight? Thanks and here it is :

1.79ct 7.93-7.99X4.78mm
crown 34deg Pavi 40.6deg
crown depth 14.5 Pavi depth 42.5
Star 50/80
HCA 0.4

 

[Paul-Antwerp]

Date: 7/14/2006 8:50:02 AM
Author: Sunshines
Hi Paul,

Sorry, but I still cant figure out the definition of tight. If I give you the real specs of a stone that scores 0.4 HCA, can you tell me what are the measurements or items here that will indicate if its tight? Thanks and here it is :

1.79ct 7.93-7.99X4.78mm
crown 34deg Pavi 40.6deg
crown depth 14.5 Pavi depth 42.5
Star 50/80
HCA 0.4

You need to have a 3D-measurement to assess tightness.

Take the very important pavilion-angle on this stone. The average is 40.6°, but what does that mean? It is the average of 8 facets, which can vary from 40.1 to 41.1, or from 40.5 to 40.6. There, you can see the difference in tightness.

If you consider that there is a drop in performance somewhere around 40.4 or below, each facet under 40.4 will bring down the performance, while the average remains the same, and the HCA-score remains the same.

And then, I did not touch the subject yet of a stone with a GIA-report, where the averages are rounded to 0.2°.

Basically, the same problem with the HCA under 1 can probably be seen in the GIA cut-research. When working only with average measurements, the stones close to the edge get de-graded, because there are too many stones (less tightly cut ones) of which a number of facets go over the edge.

Live long,

 

[strmrdr]

Date: 7/13/2006 7:09:56 PM
Author: Wink

Date: 7/13/2006 5:49:40 PM
Author: Regular Guy

In contrast, I think your comments help bring a genuine understanding to the real linearity of the HCA, which...as you say...can get lost in the execution, causing the 1 - 2 options to be the relatively safer target area.

Interesting stuff.

This has been an area of dissapointment for me as I see stones that have lower than 1 hammered as not being nice when I have them in front of me and know them to be extraordinary. I often wondered what people were smoking to say such horrible things.

Thank you Paul, for helping me also to understand this phenomina better. It is amazing how much you can know, and still know nothing...

Wink

It took a long time for Paul to hammer it into my head too.
I could never figure out if it was a problem why he was cutting them, the answer was of course that there are no easy answers and that anything under 1 isn''t good is too simple an answer.
There are some combo''s however that need to be looked over in person to see if there is a problem.
I like diamonds cut to the edges for shallow/deep because there is a sweet spot there as there is near the shallow/shallow zone if everything is cut right.
If I wasn''t buying from someone I trusted to look over the diamonds then its a different ball game and id be more likely to pass on them.
But from a trusted vendor/cutter they can be kicken.
storms 2c :}

 

[Sunshines]

Paul, Thanks very much for bringing this to light....i finally see what you mean! now I know what HCA can tell and also why it has its limitation. Appreciate it.[

 

[WinkHPD]

Date: 7/14/2006 9:54:08 AM
Author: strmrdr

If I wasn''t buying from someone I trusted to look over the diamonds then its a different ball game and id be more likely to pass on them.

But from a trusted vendor/cutter they can be kicken.

storms 2c :}

There you have stated the crux of the matter! Working with someone you trust to take the worry out of buying on the net!

Wink

 

[Paul-Antwerp]

Date: 7/13/2006 7:09:56 PM
Author: Wink

Date: 7/13/2006 5:49:40 PM
Author: Regular Guy

In contrast, I think your comments help bring a genuine understanding to the real linearity of the HCA, which...as you say...can get lost in the execution, causing the 1 - 2 options to be the relatively safer target area.

Interesting stuff.

This has been an area of dissapointment for me as I see stones that have lower than 1 hammered as not being nice when I have them in front of me and know them to be extraordinary. I often wondered what people were smoking to say such horrible things.

Thank you Paul, for helping me also to understand this phenomina better. It is amazing how much you can know, and still know nothing...

Wink

The effect of very tight cutting can also be observed in the H&A-pattern. The tighter the cut, the more crisp the pattern will be.

I think that this is what Todd of NiceIce refers to, when he says that H&A ten years ago were generally better than nowadays.

I see more and more cutters cutting on the safe side. The debate is open whether this slight deterioration in quality is observable. I do not know. Will I recognize one of our stones blindly, next to other super-ideals? Probably not. But if you give me 10 random Infinity''s, next to 10 random super-ideals of another factory, for sure, I will know which group will be ours.

Live long,

 

[Cehrabehra]

Date: 7/14/2006 11:18:06 AM
Author: Wink

Date: 7/14/2006 9:54:08 AM
Author: strmrdr

If I wasn''t buying from someone I trusted to look over the diamonds then its a different ball game and id be more likely to pass on them.

But from a trusted vendor/cutter they can be kicken.

storms 2c :}

There you have stated the crux of the matter! Working with someone you trust to take the worry out of buying on the net!

Wink

That''s one of the things I''ve run into... how do you know who to trust? I don''t just mean with my money, I mean with finding just that right stone? It seems to me like everyone here is trustworthy, but I have no idea who has access to what so I don''t know who the best choice would be for finding a particular type of stone. If I were shopping for a RB I would have looked no further than WF, but as I''ve crawled further and further back in stone-cutting time, and info becomes less and less available... I''m no longer sure what I should look for in a vendor or who is most likely to have enough access to those types of stones.

 

[strmrdr]

Date: 7/16/2006 10:06:50 PM
Author: Cehrabehra

Date: 7/14/2006 11:18:06 AM
Author: Wink

Date: 7/14/2006 9:54:08 AM
Author: strmrdr

If I wasn''t buying from someone I trusted to look over the diamonds then its a different ball game and id be more likely to pass on them.

But from a trusted vendor/cutter they can be kicken.

storms 2c :}

There you have stated the crux of the matter! Working with someone you trust to take the worry out of buying on the net!

Wink

That''s one of the things I''ve run into... how do you know who to trust? I don''t just mean with my money, I mean with finding just that right stone? It seems to me like everyone here is trustworthy, but I have no idea who has access to what so I don''t know who the best choice would be for finding a particular type of stone. If I were shopping for a RB I would have looked no further than WF, but as I''ve crawled further and further back in stone-cutting time, and info becomes less and less available... I''m no longer sure what I should look for in a vendor or who is most likely to have enough access to those types of stones.

my 2c and im gonna get yelled at:
Rounds: GOG, WF , The infinity network (Bill, Wink, Gary), niceice, jamesallen.com and others.
Princess cuts: GOG, the infinity network, superbcert, JamesAllen.com
Asschers - GOG
Cushions - Mark at ERD, GOG
Any other fancy cut - GOG

 

[Cehrabehra]

Date: 7/16/2006 11:52:10 PM
Author: strmrdr


my 2c and im gonna get yelled at:
Rounds: GOG, WF , The infinity network (Bill, Wink, Gary), niceice, jamesallen.com and others.
Princess cuts: GOG, the infinity network, superbcert, JamesAllen.com
Asschers - GOG
Cushions - Mark at ERD, GOG
Any other fancy cut - GOG

hehehe - thanks storm

 

[WinkHPD]

Date: 7/16/2006 11:52:10 PM
Author: strmrdr

Date: 7/16/2006 10:06:50 PM

Author: Cehrabehra

Date: 7/14/2006 11:18:06 AM

Author: Wink

Date: 7/14/2006 9:54:08 AM

Author: strmrdr


If I wasn''t buying from someone I trusted to look over the diamonds then its a different ball game and id be more likely to pass on them.


But from a trusted vendor/cutter they can be kicken.


storms 2c :}

There you have stated the crux of the matter! Working with someone you trust to take the worry out of buying on the net!


Wink

That''s one of the things I''ve run into... how do you know who to trust? I don''t just mean with my money, I mean with finding just that right stone? It seems to me like everyone here is trustworthy, but I have no idea who has access to what so I don''t know who the best choice would be for finding a particular type of stone. If I were shopping for a RB I would have looked no further than WF, but as I''ve crawled further and further back in stone-cutting time, and info becomes less and less available... I''m no longer sure what I should look for in a vendor or who is most likely to have enough access to those types of stones.

my 2c and im gonna get yelled at:

Rounds: GOG, WF , The infinity network (Bill, Wink, Gary), niceice, jamesallen.com and others.

Princess cuts: GOG, the infinity network, superbcert, JamesAllen.com

Asschers - GOG

Cushions - Mark at ERD, GOG

Any other fancy cut - GOG

No yelling, but...

I think you are short changing some of the very vendors you mention for rounds and princesses for not acknowledging that the qualities that make them capable of earning your trust for rounds and princesses might make them capable of assisting folks to find very wonderful stones of other shapes as well.

I also acknowledge that hard work in one area wont necessarily result in competence in other areas, but I think you have unfairly narrowed the field too far by only accepting one vendor for most of the other shapes.

Wink