HCA Score - What does it really mean?
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by Garry H (Cut Nut) » October 4th, 2006, 9:31 pm
HCA has 4 components
light return fire scintillation and spread each gets a numeric score www.diamond-cut.com.au to learn everything 1.4 is good 1.00 is good 3.00 id not so good
Garry Holloway FGAA DipDT
HCA and Ideal-scope developer http://www.ideal-scope.com and http://www.HollowayDiamonds.com.au |
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Below 2.0 would eliminate poor performers. But I do not think 1.0 will out-perform 1.4 because out of the 4 categories, I think some conflicts the other so essentially there are no "perfect scores". It depends on what you want. Some prefer more fire, some prefer more scintillation etc etc... |
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My turn. A bit silly, with the author here, but we don't have to pay per character entered...
Spidey, I don't think you have to choose those characteristics, one over the other, (although, all things being equal, sight unseen, I would select 3 excellents and a vg over 3 vg and an excellent, with all between 0 - 2), and Belle, though you're accurately saying back what Garry says, neither of you have clearly enough, to my taste, have said why. I think that although HCA is clearly linear in nature, based on the physical properties of light, I think once you get into the 0-2 zone of excellent, it's thought that too many local circumstances overtake actuals of light performance to allow you to fairly guess whether a .5 or a 1.5 will actually look better, even though those same properties are still working to accurately suggest that otherwise, a .5 would be better than a 1.5. So, big hairdos and other things blocking light as you look at the diamond, at that point, apparently, take greater precedence over angle combos, per se. ETA (but not edited to add) may factor in here. Calling 1 - 1.4 a safer zone is possible but I don't know that there's much to it. I'd keep screening for 0 - 2, and look at the accompanying range of factors. |
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i have explained, in depth, my views on hca in the past ira and simply choose to not do it yet again here. there have to be at least a hundred threads on this subject!  it would be easier to do a search, or simply read garry''s link (it''s the creators history! nothing was left out) than to keep repeating the same information over and over (and over)hca uses preset angle combinations to determine a ''score''. you can make it as complicated as you like but the bottom line is, the difference between 1.0 and 1.4 may be as simple as the pavilion angle being 40.5 instead of 40.8. is one better? |
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The hca is an quick initial filter.
It is a quick way to sort out stones that have the potential to perform well. Note I said potential it isn''t the final answer. How it does this is by telling you if the average crown angle works well with the average pavilion angle. Notice it says average.. a stone that has wide variations will score the same as a tight stone with the same averages. My general rule anything 1-2 passes and that some over 2 and some under 1 will pass. There are some combos under 1 that can have problems close up which needs to be checked. There are some combos over 2 that while they have some leakage under the table due to how human vision works it wont be eye visible. This page has more info about the under 1 issue: http://diamonds.pricescope.com/ideal.asp Once the hca filter is passed its time to move on to other info. An ideal-scope image(and/or similar scope images) or an aset image is likely the best next step. The heart images if its being sold as h&a unless you accept the h&a guarantee some vendors provide(there are some id accept but would likely want the images anyway cuz they are kewl and im paying for em in the added h&a premium). From there its helium/sarin reports and or gem files.(prefer helium scanner based files) Then its other tools such as B-scope, isee2 and others that some vendors provide. A skilled vendor who knows high performance diamonds is worth their weight in gold in helping select the best diamond. For the top vendors the tools are just a way of showing you something they already know in an understandable way. Each person has to decide what information level they are comfortable with and which vendor they are comfortable with. The trap is relying too much on one piece of the puzzle or using the wrong vendor. |
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use the HCA to help weed out stones...aka you may not want to consider a 4 scoring stone, but under 2 it's really about preference. some say under 0.6 that there is a loss of contrast within the stone, so 0 is not the most ideal score really. do you see a diff between 1 and 1.4? i doubt it. you might see a difference in the pav angle that might get you a 1 vs a 1.4 but you may not either. under 2 it's just imperceptible changes. i wouldn't toss a great 1.4 out to keep looking for a 0.9 or similar.
brian at WF has said his sweet spot on the HCA is a 1.0. i have had stones scoring all throughout 1-2. they all were a little different looking from each other. but all of my H&A's have scored under 2 and been visually amazing. i would say with the specs you posted (which are great for me) and a 1.4 score, if you have seen an idealscope with light return measurements, then you probably have found a winner. |
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I was just trying to address the OP''s question as stated. Storm, I see you agree with my understanding of the cost of character count. Belle, forgive my table manners.
On point, Storm, you and Garry think your answering the question:
with this text: "Diamonds that rate below 2 (red on the chart) are unlikely to show too much leakage darkness, overly thin or thick girdles, or fish-eyes. But there are other negatives that HCA can''t predict. Additionally, if you examine diamonds from closer up than 10 inches (25cm) in your selection process, you will probably prefer diamonds with proportions that fall in the “Young Peoples Rings” area that is marked on the chart below in the upper right of the red zone. Shallow stones (lower left on the chart) look darker if you have excellent close up vision because your head obstructs more light sources which makes a shallow diamond appear darker. But shallow diamonds have a bigger spread, and are great for pendants and earrings, where normal social viewing distances apply." And although you could make inferences, hand wave, and Garry''s separately suggested I''m smart enough to put it together...well, I''m not, really. Not sure why those lower numbers are associated with the shallow character of the stone. Not sure how necessary this would be, if one could reliably, as argued here, use HCA as a screening tool, though that''s how it''s used best. Real world, long distance, buyers won''t be sending back & forth that many options, so you do want to nail it as best as you can, first shot. Of course, send it back if you don''t like it. Compare liberally. Of course, this is only and all about understanding how the tool can be optimized. Would anyone argue with that? |
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i understand. i am just frustrated with answering in depth, the same question over and over. the answers about hca are all here. there is nothing (i am very sure NOTHING) that has not been covered. over and above the technical answer of 'is 1.4 better than 1.0' there is the more important aspect of which an individual would prefer. no formula, machine, crystal ball, tea leaf or state of the art measuring device can determine what one buyer will perceive as being 'better'. and that is probably the most important point here.  with diamonds, there is a certain range of angle combinations that perform better in general. just like anything else, once you get to the 'extremes' (<.5 for example) you are going to notice a change in performance. does this mean the diamond is 'worse'? no, it just means it's different. you may prefer it, you may not. the hca is handy for letting you know at a glance where on the performance scale a certain angle combination falls. hca is very useful but obviously it has it's limitations because it is based on angles. that's it. you can't make definitives out of a tool that never actually sees, touches or even allows input of certain optical charateristics and relies solely on accurate input from the user. we won't even go into the accuracy of the actual measurements and the effects of rounding. suffice it to say, hca is useful but it is in no way definitive. and beauty, as well as 'better' is in the eye of the beholder. |
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Everyone keeps saying, "use you eyes".
But let's face it, I'd guess 90% of Internet diamond sales happen. . . well . . . over the Internet. We aren't there. We can't use our eyes. We depend on the 4 Cs, lab reports (even reports on which lab is more dependable) tools like the HCA score, Gemscope, ISee, and branding like ACA. |
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Though I'd welcome Garry's input, too, a few thoughts (sorry...):
A hunch...is that the devil's in the details...that the good answer is in the arithmetic/calculus involved in working the math on the HCA. Really, probably, once it's answered once and for all, with a level of detail sufficient for the purpose, maybe the question will cease returning, and Belle and Garry won't be particularly bothered. This is a guess. Alternately, maybe it's rote and boring stuff, and not worth reviewing in substantial detail. I've got to say, my (edited for correctness) 6th grader is doing some pretty sophisticated statistics right now, and without furrowing my brow, I will be unable to help him (so fortunately, my wife can help better). Possibly, it's all just right here, in Garry's original HCA site. There, or within a few screens before. There, before, where he talks about method, he addresses the high level considerations about the factors that going into building the HCA. Without the arithmetic details, the how it's built may be lost, in the fundamental sense. My intuition...is that it's built either inductively or deductively. Even then, though, it may not matter. My supposition...and this is consistent with what Garry's modest prejudice he frequently reviews, such as in the recent set of videos...is that the target of the line that travels his chart that goes north west to south east is a line that represents a slightly shallow stone...making that target equal to 0 in value. Coming out from the target, you approach 10. And as you're close to the target line, with a distance of 0 - 2, you're good to go. This being the case, the distance of 1-1.9 might more typically represent a less shallow, more traditional stone. This could account for the many posts concerning problems with 0 - 1. Not sure if this helps. I am curious if the idea hangs together. Regards, |
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calculus!?  say it ain''t so! i''ve put forth my (obviously not unilateral, since there is most certainly no calculus involved) version of hca, which is the bta (belle temperature adviser) here before. hey, i''m very temperate, what can i say. temperature gets my attention! anyway, i put it together on the premise that between 60 and 90 degrees farenheit there is a range of comfortable temperatures. to get the temperature score, you simply input the temperature, humidity (pavilion, crown angle?) time of day (depth?) and amount of sun (table?) and you will get a score. depending on the combination, the score will be different. i can tell you (since i am the creator) that 85 degrees, 10% humidity, 1100, full sun will get a 1.0.  and 84 degrees, 20%, 0900, full sun will likely get a 1.4. likewise, 88 degrees, 40%, 1200, full sun will get a .5. now, keep in mind, bta does not (and cannot) include important information such as breeze (minor facets?) and full shade (optical symmetry) which would obviously change the overall effect. what bta does, is give information about temperature conditions that most people find favorable. some people like 1.0, some like 1.4, some even like .5 but you will not find too many that prefer the extremes. of course a nice breeze or some shade can help out and make an already wonderful 1.0 really good and likewise do the same for a boderline 2.2. i don''t mind 92 degrees, 10%, 1100, full sun, as long as there is a nice breeze and some shade! just don''t use bta to try and pick a ''better'' temperature condition. it wasn''t set up to do that.
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hehehe...you heard it here first!  sure, we can add averages strm. just for you. as long as you get all of your readings from channel 7. they are by far the most accurate and reliable.
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You laugh now. A follow up post will be required to a) correct the negative concept implications projected onto the (possible?) fall-out from this thread, which would b) correct mis-impressions. However, meanwhile I give you movie idea #2 (following my movie out-take #1 from the Perfect Storm) Interestingly, when going to google it, the site that reliably comes up, does so again, and featuring the signature line, by itself on the front page, that I was looking for.... -------------------------------------------------------------------------------- John Keating: Close your eyes - closure! Close 'em! Now, describe what you see. Todd Anderson: I... I close my eyes, and this image floats beside me. John Keating: A sweaty-toothed madman. Todd Anderson: A sweaty-toothed madman with a stare that pounds my brain. John Keating: Oh, that's *excellent*! Now, give him action - make him do something! Todd Anderson: His hands reach out and choke me. John Keating: That's it! Wonderful, wonderful! Todd Anderson: And all the time he's mumbling. John Keating: What's he mumbling? Todd Anderson: Mumbling truth... Truth's like a blanket that always leaves your feet cold. John Keating: [some of the class start to laugh] Forget them, forget them! Stay with the blanket. Tell me about that blanket! Todd Anderson: Y-You push it, stretch it, it'll never be enough. You kick at it, beat it, it will never cover any of us. From the moment we enter crying to the moment we leave dying, it will cover just your head as you wail and cry and scream! [class appauds] John Keating: Don't you forget this. With warm regards, P.S (eta) don't forget... |
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Though I prefer not to quote myself, in this case, I''m still at least a bit curious. Is this wacky? Or exactly!? (btw...it''s Friday!) I have 2 1/2 ideas for you as a follow up.... 1) From Garry''s HCA page(s).... "Brilliance was assessed by comparing DiamCalc Firescope and Gilbertsonscope images within each table sized grid in columns of crown angle and rows of pavilion angle. Each was subjectively rated and each virtual stone compared to the others with that table size. Stones with the least leakage and the brightest images were given a score of zero, and the worst a score of four. The author then compared images with the same values from different areas on the grid and adjusted scores for consistency where necessary." 2) From Seinfeld... Bosco The Secret Code: "George: I am not giving you my code. Kramer: I''ll bet I can guess it. George: Pssh. Yeah. Right. Kramer: Oh, alright. Yeah. Uh, let''s see. Um, well, we can throw out birthdays immediately. That''s too obvious. And no numbers for you, you''re a word man. Alright, let''s go deeper. Uh, what kind of man are you? Well, you''re weak, spineless, a man of temptations, but what tempts you? George: Huh? Kramer: You''re a portly fellow, a bit long in the waistband. So what''s your pleasure? Is it the salty snacks you crave? No no no no no, yours is a sweet tooth. George: Get out of here. Kramer: Oh you may stray, but you''ll always return to your dark master, the cocoa bean." 3) could be continued... If there is a basis for this idea, although I think it does help a lot to understand why scores of 0 - 1 come up with confounds (and as I read the HCA site from Garry, I am led to believe, really, that he accounts for these irregularities concerning big head size and such in the HCA formula itself, so I''m not seeing that you should need to do some extra math outside of the HCA to account for it), it could further be asked if it is a big or little deal. I think it is little, and would give an example of a police interrogation room, if anyone was interested, but I think the veracity of the idea needs review first. Regards, |
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guys the most of the data comes from look up charts for light return, fire and scintillation.
Spread invloves a little math calculation - but not calculus. So too fro fisheye and chipping issues penalties - like girdle thickness calculations related to crown angle - lower crown and thicker girdles should not be penalized etc
Garry Holloway FGAA DipDT
HCA and Ideal-scope developer http://www.ideal-scope.com and http://www.HollowayDiamonds.com.au |