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HCA question

P: 6/1/2004 1:21:13 PM
quaeritur Ideal Rock Total Posts: 1,238 Last Post: 2/3/2005 Member Since: 3/12/2004	I've been running stones thru the HCA for months now, in the quest for my perfect diamond. Today, I realized that I'm not sure how to interpret part of the clarification under the HCA chart: "The red area on this chart represents the lowest HCA scores. Stones near the center of the red region, those with the lowest scores, are often the least affected by small symmetry variations. A shallower stone, on the lower part of the chart, will look darker when viewed from close up, they are not for everyone. Shallow stones have the advantage of a bigger spread. They are better suited for use as pendants and earring stones where they are not usually viewed from very close proximity (a close observers head obstructs light sources that would otherwise be returned). Deeper proportioned stones, near the upper part of the red area, have more leakage. Leakage means reduced light return.[etc.]" I think this means that a shallow stone would be along the bottom of the entire chart, as opposed to along the bottom of the orange/red area. Is this correct? Thanks, quaeritur
Posted: 6/1/2004 1:21:13 PM

There are 8 replies to this message. There are 8 replies on this page.

P: 6/1/2004 1:24:07 PM
quaeritur Ideal Rock Total Posts: 1,238 Last Post: 2/3/2005 Member Since: 3/12/2004	I hope this image comes thru (and hopefully I'm not violating any copyrights/etiquette by copying it here). I've placed an X approximately where one stone I'm considering wound up after running the #s. My question -does this qualify as a shallow stone? Thanks again, quaeritur
Posted: 6/1/2004 1:24:07 PM

P: 6/1/2004 3:50:33 PM
valeria101 Ideal Rock Total Posts: 14,048 Last Post: 4/30/2006 Member Since: 8/29/2003	Did you try to input a set of crown % and angles and vary depth to see what happens ? If you really have time on your hands, you may wnat to do the same using % too Ana "The greatest experts are only as good as the sum total of what they have seen." [Souren Melikian]
Posted: 6/1/2004 3:50:33 PM

P: 6/1/2004 4:03:38 PM
quaeritur Ideal Rock Total Posts: 1,238 Last Post: 2/3/2005 Member Since: 3/12/2004	Ok, maybe I should rephrase my question... How shallow is shallow? In other words, how shallow before head obfuscation becomes noticeable? And in a "shallow" stone, are we talking about a combination of shallow crown and shallow pavilion angles, or only pavilion or only crown? Sorry, even though I've been studying up on cut for a couple of months now, I've never been particularly good at math, so this is a challenge. Hopefully, I'm not asking a totally retarded question here... Thanks for the input! quaeritur
Posted: 6/1/2004 4:03:38 PM

P: 6/1/2004 4:08:54 PM
valeria101 Ideal Rock Total Posts: 14,048 Last Post: 4/30/2006 Member Since: 8/29/2003	See? Once you use those angles, depth only determines "Spread". This gets factored into the total HCA score, but the other grades for brilliance, and so forth remain just as good. If you make the total depth too much or too little (66% or 58% for the specs above) you get a warning that the stone "appears small" or "girdle dangerously thin". The story changes if percentages are used - in that case, the HCA will try to compute the angles using that depth and all scores will change accordingly. One more reason to use angles, not percentages, if you can.. The comments you took from the HCA newsletters (or Garry's site), do not reffer to "shallower stones" for a given set of crown and pavilion angles. I guess that is a generic rule of thumb which takes into account what angles are likely found on "shallow" stones. At any rate, to judge the effect you are talking about, one would need to get into more detail about a stone than the HCA allows (Ideal Scope, and... asking Garry I suppose... Ana "The greatest experts are only as good as the sum total of what they have seen." [Souren Melikian]
Posted: 6/1/2004 4:08:54 PM

P: 6/1/2004 4:14:39 PM
valeria101 Ideal Rock Total Posts: 14,048 Last Post: 4/30/2006 Member Since: 8/29/2003	---------------- On 6/1/2004 4:03:38 PM quaeritur wrote: Ok, maybe I should rephrase my question... How shallow is shallow? In other words, how shallow before head obfuscation becomes noticeable? ---------------- Good Q... just one more detail that never got set into numbers around here "Shallow" usually means below the AGA idea range (58.3% if I remember right). But what you say, I think, didn't have allot to do with total depth but with the crown angle and height. Wasn't the effect explained relative to some Iscope image rather than set of stats? I'll try to find the relevant Newsletter... do you have a link? Ana "The greatest experts are only as good as the sum total of what they have seen." [Souren Melikian]
Posted: 6/1/2004 4:14:39 PM

P: 6/1/2004 5:39:04 PM
quaeritur Ideal Rock Total Posts: 1,238 Last Post: 2/3/2005 Member Since: 3/12/2004	Aaaaaahhh, OK, I'm starting to see what you mean. So when it says "A shallower stone, on the lower part of the chart, will look darker when viewed from close up," it refers to total depth? (by the way, I pulled that from the cutadvisor link after running the #s thru). quaeritur
Posted: 6/1/2004 5:39:04 PM

P: 6/1/2004 5:56:28 PM
Garry H (Cut Nut) Ideal Rock Total Posts: 11,577 Last Post: 11/22/2009 Member Since: 8/16/2000	Ana it is confusing to introduce %'s. If people have angles then use angles. The total depth % calculates the stones girdle thickness - that is all. and girdle thickness effects spread more than anything else. Shallow is the lower left corner. You can have a shallow stone with a 41 degree pavilion, or a shallow stone with a 36 degre crown. Shallow is a combined effect of crown and pavilion angles. You can even have a shallow (appearance wise) stone with an extremely thick girdle. Garry Holloway FGAA DipDT HCA and Ideal-scope developer http://www.ideal-scope.com and http://www.HollowayDiamonds.com.au
Posted: 6/1/2004 5:56:28 PM

P: 6/2/2004 6:34:48 AM
valeria101 Ideal Rock Total Posts: 14,048 Last Post: 4/30/2006 Member Since: 8/29/2003	---------------- On 6/1/2004 5:39:04 PM quaeritur wrote: 'A shallower stone, on the lower part of the chart, ---------------- Why do you say shallow stones are in the lower part of the chart? Just because pavilion angles are less there? Both angles represent a quite narrow interval, not enough to get you from "deep" to "shallow" cuts and back. And then for pavilion angles you have half-angle units rather - half as much as for crown angles. I guess I should have skipped those percentages... All I wanted to say with them was that one can talk of "shallow" or "deep" stones using HCA results only using %, not angles. This, just because the girdle thickness compensates a couple % of depth easily when angles are introduced. You ca say this, Garry, from authority. I thought I'd hint to some proof. Ana "The greatest experts are only as good as the sum total of what they have seen." [Souren Melikian]
Posted: 6/2/2004 6:34:48 AM

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