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HCA numbers

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Talonnav

Shiny_Rock
Joined
Mar 4, 2003
Messages
110
Can anyone explain this quote about the HCA:
"A score below 2 (Excellent) means you have eliminated known poor performers (more than 95% of all diamonds). Your own personal preference may be for a diamond with an HCA score of 1.5 more rather than one with a lower score of say 0.5"

Why would a diamond with a 1.5 be better than one with a .5? Doesn''t that kind of defeat the purpose of the scoring system?

Doug
 

dave57

Rough_Rock
Joined
Mar 6, 2003
Messages
6
I don't think anybody is saying that a score of 1.5 is better than 0.5. It's saying that you might prefer the characteristics of a stone which happens to score 1.5 to the characteristics of a stone which happens to score 0.5. The quote is just emphasizing that anything that scores <2 should be an excellent diamond.

Similarly (but not quite the same)... Most people would say that a D colour diamond is better than an H colour, but some people prefer the warmth of the H.

Just my 2¢
 

niceice

Brilliant_Rock
Joined
Jan 29, 2003
Messages
1,792
Tallonav, we can't speak for Gary or Leonid, so we don't know "exactly" what they were trying to say, but we "think" they are referring to the fact that if you were comparing various diamonds side-by-side you might choose one over another because of a difference in the brilliance, dispersion or scintillation rather than selecting the diamond that happened to score the best... Let's start with this, punch the following into the HCA

Total depth: 60.0
Table: 55
Crown angle: 34.5 degrees
Pavilion angle: 40.75 degrees

You'll get Excellent straight down the row and a Total Visual Performance of 0.9 and you might think "Oh wow! This is the diamond for me!" but what you are not able to see is how the diamond has been faceted... And the manner in which it has been faceted, the length and width of the facets as well as the consistency of their shape will have an effect on the visual performance of the stone... The HCA calculates potential based on the proportions indicated on the lab report or Sarin / OGI results, but it is just the beginning of the puzzle...

Just for kicks, change the pavilion angle to 40.9 degrees. The TVP will drop to 1.4 and you'll see:
Excellent
Excellent
Very Good
Excellent

it is unlikely that you would detect a visual difference between the two diamonds with just your eyes assuming that all other things like outside diameter, color, clarity and the quality of the cut (faceting) and proportions) were the same... But assuming that the facet structure of the stones was even slightly different, you might prefer the look of the diamond with the pavilion angle of 40.9 degrees because it happens to play with the light a little different.

Drop the pavilion angle to 40.6 on the HCA data you entered above and you get a TVP of 0.6 which from a numbers point of view might make you think that you've found a high performer because all the fields return to Excellent, but here again it will be how the diamond has been faceted that will likely contribute to your decision...

The best way to make this point is to help you visualize it...



All three of these diamonds have the same outside diameter and depth, the same table size, the same crown and pavilion angles... From a numbers point of view, they would all score the same, however they are clearly cut differently. Thus you might prefer the diamond with a HCA score of 1.3 as opposed to the one with a score of 0.9

So how do you make this determination when buying a diamond on-line? Work with a dealer who has the diamond in their possession as opposed to one who is merely selling diamonds off of a list and having them drop shipped from their supplier... The dealer in possession of the diamond can provide you with the additional data regarding the diamond that can only be determined by being able to see the diamond.
 

pricescope

Ideal_Rock
Joined
Dec 31, 1999
Messages
8,266
----------------
Can anyone explain this quote about the HCA:
"A score below 2 (Excellent) means you have eliminated known poor performers (more than 95% of all diamonds). Your own personal preference may be for a diamond with an HCA score of 1.5 more rather than one with a lower score of say 0.5"

Why would a diamond with a 1.5 be better than one with a .5? Doesn't that kind of defeat the purpose of the scoring system?

Doug
----------------
There are several reasons, Doug.

HCA doesn't take all the diamond cut factors into account (e.g. symmetry, minor facets, etc) but only average crown and pavilion angles. Therefore, diamonds with the same average crown and pavilion angles might look different and different viewers might prefer different looking diamonds. Some could be brighter (reflect more white light), other less bright but have bigger splashes of color.

Beauty is in the eye of the beholder.

Same applies to Brilliancescope and Firescope/Ideal-Scope. If a diamond shows no light leakage in idealscope, alone it doesn't guarantee that you will prefer it to a stone with slightly visible light leakage.

If one diamond scores slightly higher on Brilliancescope than another, you still might prefer the look of the second one simply because no machine can tell you what YOU would like the most.

Besides, white light return is not the same as human perception of brilliance. See /www.pricescope.com/msu/perception.asp>.

People look with two eyes, diamond and people eyes are not still but in constant motion. Color of your hairs, clothes, room, etc. will affect diamond appearance. Your vision conditions, distance from your eyes to the diamond, and so on, and so forth....


That's why Garry's wrote "Your own personal preference may be for a diamond with an HCA score of 1.5 more rather than one with a lower score of say 0.5".

Use all these tools to narrow down your choice and then use your own eyes as a final judge.

Hope this helps.
 

Talonnav

Shiny_Rock
Joined
Mar 4, 2003
Messages
110
I think I've got it now. One diamond I was looking at last night was a good example. I was just going thru a list of diamonds which all consistently had an HCA of .9 to 1.1. Then this one diamond had a HCA of 1.5. It's fire & sparkle on the B-Scope were off the chart!! The brilliance was a border line High - Very High.

Guess that proves the point that all diamonds aren't alike.
 

dbretton

Rough_Rock
Joined
Dec 1, 2002
Messages
50
I believe that some 8* diamonds (reputed to be the best of the best), have scored 1.x on the HCA.

I have seen a diamond score an exact 1.5 on the HCA outperform (to my eyes) a diamond which scored an exact 0.5 (both E, both id/id/id, both AGS, one SI1, one SI2, both 100% eye-clean).

In my wallet, I carry around a small printout of something Robin/Todd over at niceice had posted here one time. They posted the proportions for a great stone.

Total Depth: 59 - 61.8%
(the lower the better)
Table Diameter: 55-56%
Crown Angle: 34.3 - 34.8 degrees
Pavilion Angle: 40.5 - 40.9 degrees
Girdle: 1 % thin to 1.7% medium, preferably faceted
or at the least polished.
Culet: GIA none or AGS ideal.
Proportions: it's in the bag with the guidelines set forth above...

I keep this with me all the time. I don't feel the need to resort to the HCA.
 
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