Big difference in HCA reading for similar stones

[my2cents]

Hello,

Been reading the various posts and its all very interesting. Looking for a diamond at the moment and found 2 stones of very similar stats and price but with very different HCA reading! They are EGL rated and so stated pavilion and crown in % which is not so helpful... Can anyone explain?

Stone 1

Depth: 62.4%
Table 56%
Crown 15%
Pavilion 44%
HCA 4.3

Stone 2

Depth: 62.3%
Table 56%
Crown 15%
Pavilion 43%
HCA 1.0

Many thanks for your expert opinions.

 

[Maxine]

Not an answer , but another question: How would the idealscope images look for each of these stones???? Just curious.....................

 

[Garry H (Cut Nut)]

The second stone probably has a thinner girdle
but will look better

 

[RockDoc]

I understand your problem...

Diamonds are each a unique creation. Two stones with similar characteristics can look very different.

Proportional numbers are averages... and don''t always predict how a stone will look.

SO many people rely on numbers which is sort of the appetizer rather than the main course.

After all what do these numbers represent? Light return.

Isn''t it more advantageous in your analysis to rely on the Brilliance Scope which actually reports the light return, over a computer program that hasn''t seen the stone?

While proportions do have in many diamonds significant reliance for some, they don''t always guarantee that the appearance of the stone, will please you as much as the numbers do.

What has greater significance to a purchaser, How good the numbers are, or the visual appearance of the diamond, where the diamond itself is the sample, rather than a prediction made based on a computer generated formula ?

Rockdoc

 

[valeria101]

The darn 1% of pavilion makes a difference of about 1 degree on the pavilion angle - it seems that this is a significant number for gem cuting (incredible as it may sound).

Using %, the girdle size and culet size begin to influence HCA results and those are not precisely measured on certs. You were lucky enough

to find just the right example.

Between the two, I'd bet on the one with the high score allright - just because I couldn't tell what exactly accounts for the unlucky deep pavilion of the other. If there were IdealScope (or photos , or any other kind of reasonably standardized image taken of the stone, not using numbers) around, then much less guessing would be needed.

Just my 0.2 worth IMO, as usual

 

[Maxine]

Can anyone post examples of what the two IS images might look like?

 

[Regular Guy]

I ran the numbers just to double check your work, and of course found the same...previous attempts to use the %s by me must have been luck, as this example points out.

RockDock, unless you''re just a shill for the Brilliancescope that you use, what do you have to say about standards that have been broadly accepted by your peers, that use measurements all the time, including AGS for starters, and not to mention Ana''s keen recent observation, that it''s the angles that motivate the cutters at the outset.

Although my recent insight was to invoke Lostdog''s observations on the HCA, and suggest that a particular angle of variance may be being called out with this example, again, Ana''s insight above, that a % is close to a whole degree, where both the pavillion generally, and relatedly, the preferred data capture goes to a tenth of a degree, seems to tell the story best.

So now I do understand more clearly why angles are preferred over %s; many thanks for this example!

 

[lostdog]

It's sort of all of the same thing.

My point was that with current incarnation of HCA you can't split hairs in making a comparison. Unless you are on a "hard" measured point within the graph, there is enough opportunity for variance in the method that calculates scores that you can't necessarily claim one diamond's performance is actually higher or lower than another's when the difference is small.

"Small" is a fluid term, in relation to the shape of the graph surrounding your actual point in question.

If carefully considerthe measured explicit data points that surround the nominal measured crown or pavilion angle of the diamond you are evaluating, and also consider the factors that are determining the performance that goes into HCA, you may be able to give more or less weight to a particular score, and/or to a difference between two scores.

HCA's stated original purpose was to allow a buyer (a commercial buyer at that) to rule in additional stones that might be of very high performance, yet not within AGS 0, or other traditional ranges. Maybe it's at heart a pointer towards high performance RB territory, not a stone picker. Some want to call it an exclusionary tool, one which isn't going to tell you which stone to pick, but rather which ones not to pick. Which seems true within reason. I wouldn't say averages are the issue. It returns results based on assumptions, not on actual stones, in order to rule in (or to not rule out) more good stones than previous systems. It's a starting point, and not a conclusion. Some stones are still going to fall through the cracks in the assumptions, and end up outside consideration, that's hard to avoid.

It might seem very safe to say, "yes, under 2.0, that's all I'll look at". But there are still stones that the methodology of the current HCA is probably shortchanging. These are the stones that are right at certain thresholds, the "edge of the cliff", so to speak.

In my example (from the other thread with C34.8 for the following P angles mentioned), by the time you get to P41.5, performance has no doubt dropped off. Look at enough scope images (real ones) and it's very clear.

The trouble is that most of the hairsplitting is attempted with stones that would be, in this case, P40.5 to P41.1 or so.

Some say under 40.6, performance is actually sliding already, though that shows up as the lowest HCA within that set of proportions. Clearly as P increases to 41.5 performance is slipping, too. The ultimate could really be 40.6, or 40.7, or 40.8, or 40.9, (and still nearly optimal slightly past that). It's easier to find 40.9 selling at a premium, or certainly not for less than the other angles I just mentioned.

So 1 degree difference is a potentially very significant difference if it puts you over the threshold. 0.1 degree really isn't significant by itself, if you are trying to make a stone to stone comparison.

Consider that the real stones all do not have an actual single angle for the C and P facets, but a range, and that the extent of the range varies. Some stones with 40.9 nominal and facets that vary .4 degree +/- aren't uncommon. .2 is more common. The point made about knowing the relationship between the actual pairs seems to be one of the biggest keys, probably the true isse.

Still, I find better idealscopes correlate as well to stones where the range of angles is small, and the nominal angle is in the middle of the range rather than shading toward the bottom with one or two high outliers. Not all Vendors post the facet by facet measurements, more seem to post just the basic table of numbers, typically, so this may help distinguish stones you might want to learn more about.

You can't take an HCA score too literally. To use an analogy for applying HCA, let's say you want to find a certain house. AGS cut puts you in the right time zone, while current HCA gets you into either the right state or possibly into the right area code, though what it is giving you numerically suggests the precision of a zip code.

(And separately on percentages: Percentages rounded to whole numbers and then input into HCA may be dropping too much data in the process to allow you to give the score comparison much weight unless you are dealing with extremes. Two angles off by a tenth could go through the averaging and conversion process and be compared as if they were off by as much as one whole degree. (Some numbers that are in range for one cut grade when using tenths of %, will round up to a whole number %that is in the next cut grade). )

So in the example that starts this thread, HCA 4.5 and HCA 1.8 would lead most people to say don't touch the 4.5. In reality, HCA's of these stones might be much much closer to each other.

The poles of BIC and TIC are worth considering in a comparison, but don't worry about 1.7 versus .5 or anything else within the excellent range unless both stones are on the exact hard angle combinations that were used to build the HCA scoring methodology. The other scores are approximations which may come out higher or lower than the true performance HCA would assign if it resolved performance details that fine. So also don't pay a premium solely for a slightly better HCA in the absence of other differences between stones. It's a great tool, a very valuable one, and a tool that seems to have taught many people an important lesson about visual performance, but you should know how to use it. An Excellent HCA is an Excellent HCA unless you want to get under the hood and get dirty your hands with the details.

If you are going to get into the details, it seems likely that some stones are being short-changed on HCA scores. For one, the "threshold stones" mentioned above, whose scores are potentially exaggerated to be higher numerically because of who their "hard" point neighbors are. But for another, the "trough stones", which in reality might deserve the lowest HCA's of all, but which are located at minima that fall between the "hard" points are held back, scoring numerically higher, because of this, too. Neighbor on left is a 1.7, neighbor on right is a 1.7, so everybody in between also gets assigned 1.7 even though inbetween the stones probably dip to something lower.

(also, a stone at a maximum in the curve might get a benefit, too, but hopefully you aren't looking for the true highest HCA numbers).

On the flip side, a low number HCA might be coming out slightly lower than "true" in certain cases, but not by all that much from the curves I have looked at. So if you didn't have detailed info on the facets, and you didn't have a scope picture, staying in the under 2 HCA range would keep things safer, the possible variance between calculated and "true" HCA smaller, which is why that piece of advice has become gospel for many. This means, in the example I keep drawing angles from, 40.9 P and under.

If you are going to play near the edge of the cliff, a much tighter range of angle measurements may perform very well. Idealscopes of nominal 41.0 and 41.1's can be all over the map, but if the variance of the actual facets is low, they usually look rather good. Actually, I think the scope of a tight P41.0 in this range looks significantly better than the virtual version that shows up in GemAdviser and elsewhere. The table on 41.0's has much a lighter pink for the virtual stones than the Idealscopes I see from real ones with "matching" numbers. (it's over my head technically to speculate on why the virtual one is "pinker" than the real ones, but since the true experts on this topic are around somewhere, maybe they have some input?). Nice 41.1s work, too. At 41.2, it's becoming likely some facets stray past 41.3, and the idealscopes usually start paying for it, too.

I would be very interested to see, though, something in more detail about the "threshold area". Does anybody with the real software toys want to plug in a 55% table, 34.8 or 35 crown, and run the pavilions from 40.5 to 41.5 by tenths? What are the performance numbers showing? How do the ray tracing images change? With a better idea of what this might imply for the shape of the perfromance curve, it would be a better map for the "edge of the cliff".

 

[Paul-Antwerp]

One of the problems of Pricescope is that there are so many threads, that you sometimes miss an interesting one.

The truth of the matter with these two stones is: they are NOT similar at all.

The pavilion depth of number 1 is 1% higher than that of number 2. This is not close, this is an enormous difference. Translated into pavilion angle, this comes close to a full degree of difference. Since the pavilion angle is most critical in setting up light performance, such a big difference has a substantial effect.

For those who think that a 1-degree-difference in pavilion angle seems little, years ago, when working in melee-sizes and smaller, we estimated pavilion angles up to a degree by eye only. No Sarins in those days, and shadow-measurement did not work on 0.03 Ct-stones.

At the same time, stone number 2 has a girdle that is 1% thicker (Garry was not paying attention in his reply).

And of course this has great effect on light performance, since that performance stems from the combinations of proportions. If you adapt the most important proportion dramatically, and do not adapt the other proportions, you definitely get a totally different result. Elementary, Watson.

Live long,

 

[Garry H (Cut Nut)]

You are correct paul - I was travelling and time poor - i blooped

 

[RockDoc]

Regular Guy....

RockDock, unless you''re just a shill for the Brilliancescope that you use, what do you have to say about standards that have been broadly accepted by your peers, that use measurements all the time, including AGS for starters, and not to mention Ana''s keen recent observation, that it''s the angles that motivate the cutters at the outset.


RE: Shill for Brilliance Scope....

Not sure I understand what you mean? I am an independent gemologist appraiser who uses the B Scope to inform consumers that want that report.

I am and have worked with the B Scope Analyzer and Viewer for a few years now. A shill is secretive about getting someone to buy something. I have been very open about being a Gemex equipment user, offering and promoting the service for those who wish it. How is that "shilling", as certanly I haen''t tried to prmote the briliance scope equipment secretly?

There is all this talk on PS about angles and measurements. Many times they can show the accurate information to PREDICT how a stone will perform. Many times this works, but many times it doesn''t. Programs that estimate light return that haven''t analyzed the exact stone, are based on formulas of light refraction. These may or not be accurate. Certainly they are helpful, and better than no analysis. Sarin/ AGS/ Diamond Calc/ and others are improving their programs to link with the HCA, based on the measurements of the stone''s actual imaging. This is certainly better and a far more encouraging bases for analyzing "numbers".

But what good is any combinations of numbers, if the stone is cut in such a way to have lousy numbers and look great, or great numbers and be a dud.

The B Scope instrumentation is a light return measuring device. It doesn''t care what you put in it. If you have a dead elephant''s foot than return light, it will measure it. Over and over I''ve written, as well as others, that no machine is perfect, and no program is either.

The Bscope reports information in a different way, which is based on an actual scan of the individual item. The B Scope doesn''t care what the numbers are. It is indictive of how the stone returns light based on an actual measurement. The results are repeatable, within very reasonable variances. It is a good machine for comparing the light return from several diamonds.

Another area that needs concern. The numbers for many of the reports and averages, and lower girdle/pavilion length isn''t addressed, as is the upper girdles and star facets, except on the type of reports that are based on the measurement of the actual diamond.

If you add the facets numbers reports on major lab certs, they only report the table, crown mains, and pavilion mains, which total up to 17 facets.. What about the other 40? Think their not important? They are ! For over 3 years I''ve been writing about the "40 mystery facets" as I''ve called them. Gradually, there is now being more attention paid to this, even by the new cut grading systems of AGS, and possibly GIA.

Hope this answers your question, and my clarification of being a shill.

Rockdoc

 

[Regular Guy]

RockDoc,

In support of Ana''s point on her other post...

"Interestingly, none of the cut grading systems cares to disclose their level of sensitivity. At least, not that I know of. So much for disclosure..."

Well, close to on point, maybe...not claiming to be any kind of expert, but trying to be a smart shopper, listening to experts, and increase the chance of investing well shopping while economizing on cost. AGS has built a solid reputation on asserting a system defining ideal based on the predictive elements of the principle crown & pavilion angles, plus some things, alone, and the reputation seems solid. While I would not deny your post, I''m guessing that for most of the time, it works, and as Storm frequently says, your best shot is to test both frequently and often.

Rock, rather than asserting something like: "Many times this works, but many times it doesn''t..."...what are our odds, exactly. I''m guessing they''re good enough for AGS to have built a system that depends on it (even though...well, heh, heh...I guess they''re taking it down now.)

I suppose my real point was made a couple of weeks ago...in basic agreement with what Ana might have been saying...it will be helpful to get a sense of the different weights that we can attribute all the iterated elements to, in the beauty of a diamond, from pavilion (topping the list, I''ll bet) down to your 40 + minor facts, which count no doubt. How much? I''d like to know.

 

[Garry H (Cut Nut)]

Roc would you like to run some well cut and poorly cut CZ''s on the BScope and tell us what the results are?
If people were to believe these results they might never buy diamonds again.

 

[valeria101]

Date: 3/13/2005 12:44:50 AM
Author: Garry H (Cut Nut)
Roc would you like to run some well cut and poorly cut CZ''s on the BScope and tell us what the results are?
If people were to believe these results they might never buy diamonds again.