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Why would the HCA score be low on this diamond

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pyramid

Ideal_Rock
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I do not have the full depth percentage and none of the angles but using what I have why is this diamond scoring low

Round brilliant 0.67 carat
This is in a B&M store
measurements 5.52 - 5.55 x 3.53 mm
Table 55%
Crown height 16%
Pavillion depth 43.5%
Girdle Thin to Medium
Culet Medium (although I can barely make it out with a loupe)
Polish Good
Proportions Very Good

It is an IGI Antwerp report done in 2004.


Is it the pavillion depth which is bad or what?
 
PS

Thank you for any information.
 
You can''t even get a score at all on the HCA without putting in a depth, and since you don''t have one, I''m assuming you used a default depth.

That''s really not going to tell you about this stone, so the score it got really doesn''t mean anything since it''s not sound information.
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Thanks aldjewey.. I did realise I don't have enough information but what I did was add the 43.5% pavillion to the 16% crown and then add 3% for the girdle and put in the 2 from the chart below the cut advisor for a medium culet. The scores I got when playing around with depth numbers were all about 4 or 5.

I just wondered if anyone knew if the pavillion depth was bad or the crown height or what?

Is there no way to obtain this information from the measurements of the width and depth of the stone as the other percentages are a percentage of the width.
 
The angles are often rounded and igi sometimes grade diamonds mounted making the measurements more guesses than anything but here is why hca is giving it a bad score.

prydia1.jpg
 
Thank you strmrdr. Would this diamond be a bad purchase then? Is that showing leakage under the table. Do you know what the hca score would be from this, e.g. what depth would I put in?

It is just that the report says proportions are Very Good would that be the case or would it be good. The colour of the diamond is a D also.
 
From the limited and questionable (igi) info provided unless it looked good to my eye and was a real bargain I wouldnt buy it.
An idealscope would tell the truth of the cut in 20 seconds.

Use 63.6% as the depth its close.
depth/diameter * 100
 
Date: 9/4/2005 3:34:26 PM
Author: Pyramid
Thank you strmrdr. Would this diamond be a bad purchase then? Is that showing leakage under the table. Do you know what the hca score would be from this, e.g. what depth would I put in?


It is just that the report says proportions are Very Good would that be the case or would it be good. The colour of the diamond is a D also.
Click to expand...

I would never buy a D color diamond without a GIA/AGS cert if it truely was a D color the added price they could charge with a GIA cert is what they would have done.
 
Thanks again strmrdr. I have another question I meant to post in the last one, where is says on the diamcalc report you posted -0.05 carat, is this meaning that it faces up as a 0.62 carat instead of a 0.67 carat? I expect it does.

I am getting a HCA score of 3.4 now which is better than the ones I got before but not by a lot.

Also the culet I have added as 2 which is the start of the medium. This is not showing on your diamcalc drawing, does culet not make a different on that?
 
Hi!
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There might be a problem with using the HCA liek this here...
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Since you do not have the angles and have inferred the total depth from percentages... then the HCA deduces angle measurements based on your percentages. So the total depth you calculate affects the score badly on all fronts (all four components of the score). If you had angles to begin with, than the total depth would have ony influenced spread - the other three components of the HCA score depend on table and angles only.

This is because the free online version of the HCA does not allow to plug in girdle width. I do not know what width is assumed when the percentages plugged in are translated into angles and given that a little variation there blows the total score all over the place I would not dare guess.

Same thing happens if one tries to evaluate those numbers via a GA model (like Strm did). In addition, that model requires a hoard of other details to be determined in order to generate anything close to the look of the real stone (minor facets, symmetry details... and perhaps others. So, the image in Strm''s post is not even close to the precission the GA can deliver. I don''t think it can get to be a more precise guess than the HCA (which suffers from the deadly limitations described above) because, simply put, the information is not there.

Hope this is not too confusing...
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BUT
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you do have total depth - it is the 3.53 number there among the measurements (as given: measurements 5.52 - 5.55 x 3.53 mm)

To get the percentage, dive depth by the shortest diameter. You get 63.9 (a bit much, but no sweat if the price for size is right,IMO)

So... you can use the HCA. And it may well be that using percentages is less precise, but not that badly.

The new result is around 5
2.gif
and that hovers from 4 to 6 or so if you change the culet size from 0 to 3. If you had angles instead of percentages, that would not have counted either. As is, it countsnot as a microscopic hole inside the diamond, but a factor used byt the HCA to compute pavilion angles - and those are the most important determinant of the darnscores.

As far as I understand, this stone was not meant to be judged by the HCA, or the HCA was not meant for this stone or somehting.

You may still like it, the HCA does not.
 
I understand what you are saying valeria101. The only thing is when I put it in with the percentages I get the 5 score. However when I put in either 63.9 depth percent as you said or 63.6 depth percent which strmrdr estimated along with the pavillion and crown angles I get the 3.4 score.
 
Also re the culet, would this affect the stones spread. I am just wondering if the stone looks more like a 62 point than a 67 point due to the depth, as shown on the diamcalc report.

I do think that what strmrdr did is accurate as the stone does appear to be deep under the table which I think is leakage as shown in that report.
 
Woops val is right its 63.9 not 63.6 I used the wrong number.
Nice catch Val :}

From the diamcalc manual

2.4 What is spread?
The spread is a parameter showing the difference between the weight of the given diamond and the weight of the "standard diamond". The "standard diamond" has the same area (??? maybe “weight”) as the given diamond but Tolkowsky proportions with the medium girdle thickness. The spread parameter tells you if your diamond looks more or less massive than it really is. The spread parameter also works with fancy shapes comparing them with round "standard diamond". The spread indication is available only in the registered version.

........
You cant perfectly model a diamond in diamcalc without a srn file or better yet a helium report so its only an appoximation and not that great of one.
Given that the info on an igi cert can be questioned to the diamcalc info should only be used as a very rough guess.
This also explains why some details like the girdle and cutlet might be slightly off.
In a lot of ways diamcalc gives you an idea of the best it can look with those numbers if they are accurate. It can look worse.
 
Strmdr, does this mean that if the IGI measurements were correct that the spread of the diamond may not be affect as much as the diamcalc says because the diamond has girdle width thin to medium but the diamcalc is calculating it on girdle valleys of slightly thick? Do you think the spread would still look as a 0.62 carat stone instead of a 0.67?


Another thing I was wondering as the measurements of the diamond e.g. table, crown height and pavillion depth are all within the ideal range, why is the depth not? Is this because the crown and pavillion are both in the maximum figures which means the depth is more than ideal. Do the ideal cut ranges not allow for maximum or minimum i.e. a deep stone or a shallow stone?
 
As the diamond has a full IGI report showing the plotting I would assume that it was graded unmounted. Would that be correct do you think? Otherwise how would they have been able to see the pavillion for plotting it.
 
Date: 9/4/2005 4:32:50 PM
Author: Pyramid
I understand what you are saying valeria101. The only thing is when I put it in with the percentages I get the 5 score.
Click to expand...
So... that''s that... In fact, I would not dwell much on the HCA results because they are way senzitive to the variation of cult and girdle width. We do not know those down to a fracton of percent allright (if even that is useful) and slight variations of either (say, culet from 1.7 to 3, as given in that table, and girdle from 2 to 3) throw results all over the place.

Really...

The Gem Adviser does not predict weight very acurately because numbers are missing - I would not demand to this tool to account for the last five points. Same goes for the ''spread'' measure in the HCA (I gave up looking at it some time ago). It is weird to look for ''predicted'' or estimated numbers for spread when you''ve got the diameter measurements infront
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Some non-ideal (understanding AGS0 old style) round diamonds have better spread than the ideals, and even with the ''ideal'' parameters at hand slight variations (down to 1-2 tenths of a milimeter as i see sometimes discussed) are easily accounted for by details of cut that are not reported by the usual stats (even if angles were there). The outline of the girdle and minor facets are enough to distribute volume a bit, for what that matters.

If the GA picture fits, that''s lucky. Also, the HCA and GA light return scores are somewhat more senzitive than the pictures show. Especially outside the H&A details that make Pricescope''s ''bread and butter''. I am not sure that any HCA 3 round would have a ''worse'' GA image than any HCA05 diamond. In this case there is nothing I can do further because I do not have a copy of this software on this computer.

Strm can do it better than (the sckeptical) me, anyway
1.gif
 
Date: 9/4/2005 6:03:04 PM
Author: Pyramid
As the diamond has a full IGI report showing the plotting I would assume that it was graded unmounted. Would that be correct do you think? Otherwise how would they have been able to see the pavillion for plotting it.
Click to expand...
If the stones are graded mounted there is a mention on the report.

Is this (troublesome) diamond allot better priced than one with more detailed information available? IGI is not known for the most precise grading...so if comparing with GIA or AGS a couple of grades are fair game. Just curious.

Hope some of this helps.
 
Date: 9/4/2005 5:45:57 PM
Author: Pyramid
Strmdr, does this mean that if the IGI measurements were correct that the spread of the diamond may not be affect as much as the diamcalc says because the diamond has girdle width thin to medium but the diamcalc is calculating it on girdle valleys of slightly thick? Do you think the spread would still look as a 0.62 carat stone instead of a 0.67?
Click to expand...
A diamond with Tolkowsky proportions and the same mm diameter is modeled below.
So campared to a Tolkowsky proportioned diamond it has .05cts of weight in it.

pydia2.jpg
 
Thank you valeria101. The thing about this stone is that it is at a local b&m in the UK, the jeweller said he does not work with GIA and historically has worked with HRD or IGI Antwerp. The jeweller would give me a trade in on a mall diamond I own and dislike, this stone looks a lot better than it does. The other stones he has as well as other jewellers in the area are all the same cut wise, lots of 60% tables, so I will not get an ideal cut.
 
Thanks Strmrdr, is it Tolkowski that the modern day brilliant and ACA H&A diamonds are measured by. Is a Tolkowski 1 carat the 6.5mm that these cuts are.

Oh and why does Tolkowski cut diamond show partial leaking under the table also?
 
Tolkowsky proportioned .67ct

prydia2.jpg
 
Thank you strmrdr.
 
Date: 9/4/2005 8:08:41 PM
Author: Pyramid
Thank you strmrdr.
Click to expand...

Your welcome.
 
Date: 9/4/2005 7:41:07 PM
Author: Pyramid
... lots of 60% tables, so I will not get an ideal cut.
Click to expand...
Since this is not ideal by proportions either and the local jewelers believe in 60/60... perhaps you could locate a better one by looking for a stone with better proportions among the 60/60. Less depth would not hurt either. I have a bit of doubt about IGI''s ''D'' being the same as GIA''s. HRD''s should, if that is the other alternative on the ground. I cannot remember if HRD does mention proportions on the lab report, but if you care for the cut grading thing and numbers are hard to come by locally, than you could probably use Garry''s IdealScope. Looking at diamonds outside the (now old) AGS0 bracket can yield lots of pleasant surprises. And your serach will be allot more precise than the guesstimate based on IGI measurements, as far as I can tell.

Just my 0.2, of course.
 
I agree with val 100% if these diamonds are local spend the money and get an ideal-scope and learn how to use it.
 
The only other diamond to really choose from is a G VS2 0.70 but does not look any better to my naked eye. I really like the D colour on this one. It may not be D but it does look better than the G. I really want to use this vendor because I want to trade in my other diamond.
 
at .67 Pyramid, it is q rare weight and could always be recut smaller.
It will also possibly be a new GIA Excellent under their new cut system.
 
Date: 9/5/2005 2:31:00 AM
Author: Pyramid
The only other diamond to really choose from is a G VS2 0.70 but does not look any better to my naked eye. I really like the D colour on this one. It may not be D but it does look better than the G. I really want to use this vendor because I want to trade in my other diamond.
Click to expand...

Get an ideal scope and check them out that way would be the best thing you could do.
 
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