很长,最后的总结在24楼.
版主,够精华贴不?嘿嘿...
[此贴子已经被作者于2010/7/8 17:56:40编辑过]
[此贴子已经被作者于2010/7/8 17:56:40编辑过]
[此贴子已经被作者于2010/7/8 17:44:44编辑过]
Mr. Holloway happens to be one of the very first FireScope? owners who purchased his in the mid 80's when they were first made available for sale to the general public. From noting and studying the primary angle combinations and relational table sizes (crown angles, pavilion angles and table size) that contribute to solid red/black reflector images, Garry has developed an easy way for consumers to get a virtual peek at which stones will and will not fare well through a reflector technology such as a FireScope, IdealScope or DiamXray viewer. We believe Garry has the best interests of the consumer in mind by making this tool available to the public to use for free. I know a lot of time and effort has gone into putting it together and to make it available to the public for free can only be applauded.
So, in essence ... a great HCA score on a diamond = a diamond that will get an excellent reflector image through one of the scopes/devices mentioned above and apparently a more beautiful diamond. This is indeed the case in many instances but there are many other instances in which it is not which we will examine more closely as we delve further into this study.
For a cursory understanding, looking at the 2 red reflector images side by side it shouldn't be too difficult to figure out which diamond should get the better HCA score. A quick simple interpretation of red reflector images (a more exhaustive article on interpreting reflector images can be found here) is you want to see a good saturation of reds with a good distribution of blacks. White = light leakage so the less of it the better. The grading system starts at 0 and goes to 10 on what is supposed to be a linear scale. 0 being the best with 10 being the worst. HCA Scores between 0-2 are deemed as the rarest and best.
HCA Score = 6.7 | HCA Score = 1.5 |
Total depth: 65.1% Table: 63.2% Crown angles: 38.6° Pavilion angles: 41.7° |
Total depth: 61.1% Table: 56.6% Crown angles: 34.9° Pavilion angles: 40.8° |
![]() |
![]() |
![]() |
![]() |
So ... for the most part this is all well and good eh? This assessment is accurate as we've personally confirmed it with a live analysis of the diamonds in question. Here is a photograph taken of these 2 diamonds in simulated diffuse daylight (GIA DiamondDock?) which plainly demonstrates the optical characteristic of brightness. The difference is quite easy to see regardless of the size difference between these 2 stones.
With this particular example the same is true regarding the optical characteristics as observed in spot lighting.
Here is a photograph of the same 2 diamonds taken in a spot lighting environment.
To view a video of this comparison in diffuse daylight and spot lighting for a more accurate assessment click this link.
The preamble to the HCA states "The HCA estimates a round diamonds appeal based on its potential ...
A diamond getting a score of 0-2 is deemed to be "Excellent" and while it does state that most people prefer stones scoring in the 1-2 range, we find that most consumers natural tendency is to get the best HCA score they possibly can. As both a bricks and mortar store and an online retailer of diamonds many requests online come in for scores between 0-1 as these folks believe they are being even more selective. You will soon learn this is not necessarily the case regarding HCA scores and why.
Using the HCA requires the user to get some basic information about the diamond they are considering. Namely ...
Below is a screen shot taken off the PriceScope website of the HCA software tool showing the user where to input the data.
Prior to January 2006 you could not get this proportion information (primarily crown and pavilion angles) off of most lab reports and one would need a Sarin, OGI or Helium report which gave the measurements you needed to input into the HCA. As of January 2006 all GIA Reports include crown and pavilion angle data as well as other information needed to get HCA scores. While GIA does a small amount of rounding in their measurements it is generally not enough to impact the face up appearance of a diamond although it could dramatically impact the HCA score. (Note: The GIA Excellent Cut Grade does cover a certain variety of appearances which we further discuss in our article on the subject). Below is a chart showing diagrams on the various reports generated for diamonds wherein you can get the total depth, table size, average crown angles, & average pavilion angles needed to get an HCA score. On each of these diagrams you can find total depth %, table %, crown angles, pavilion angles and culet information.
Portion of New GIA Report showing proportion data |
|
Portion of an AGS Report showing proportion data | ![]() |
One Sarin Report showing proportion data | ![]() |
Portion of OGI Report | ![]() |
Portion of Helium Report | ![]() |
On the surface the HCA makes a lot of sense because there are many instances where diamonds that have great reflector images are indeed beautiful looking diamonds. The 2 example diamonds we showed above is a classic case in argument for the HCA. Garry has done a great job of establishing cut off points on the HCA for when leakage under the table becomes present in diamonds too. For sake of example we'll be using live stones and also simulated diamonds and graphics in DiamCalc to demonstrate this.
Recently we acquired the diamond below on the left which is a 1.01ct I VS2 which has a GIA Cut Grade of "Very Good" because of a few reasons (very thin girdle, good symmetry are two reasons), but the one of note being a proportion set which contribute to a decrease in optics in the GIA system. When we ran the diamond through AGS' official Performance Grading Software it scores an overall "1" for Light Performance taking a hit on the "Tilt" factor. While this diamond does not make ideal grade for Light Performance in both GIA and AGS Cut Grading systems, it scores a favorable 1.5 on the HCA.
Here is a photograph under the GIA DiamondDock of this shallow angled combination alongside another diamond which also gets the GIA "Very Good" grade which has steeper/deeper angles. Two varieties of GIA Very Good's. In our observations the shallow angled stone on the left has the brighter appearance as we can observe greater brightness and contrast in it. The HCA scores for the following 2 stones are beneath.
[此贴子已经被作者于2010/7/8 17:46:58编辑过]
Many ideal cut diamonds on the market today have crown angles in the 34.x° to 35.x° (although there are quite a few proportion combinations that produce ideal optics).; When you couple those crown angles with pavilion angles at or over 41° leakage faintly begins to occur under the table.; Here's an example generated within the DiamCalc software showing it.; The diamond on the far left; has ideal proportions and makes top grade in any system or technology.; In the center stone we adjusted the pavilion angles from 40.8° to 41° and you can note more light pink as opposed to dark red under the table facet and the stone on the far right is beginning to get whitish under the table with a 41.1° pavilion angle; White = blatant leakage which generally occurs as angle relationships tend towards the steeper/deeper side.; Note the decrease in the; HCA scores as we adjust the pavilion angles from 40.8° to 41° to 41.1°.
Ideal cut diamond with a 61% depth, 56% table, 35° crown angles and a 40.8° pavilion angle. | Same as left but with a 41° pavilion angle. Note how the reds get slightly more pale under the table facet. |
35.1 crown angles, 41.1 pav angles & a 56% table.; Leakage is more apparent under the table as observed in red reflectors. |
![]() |
![]() |
![]() |
HCA = 1.5 | HCA = 2.2 | HCA = 3 |
So ... the HCA sees this faint leakage under the table of the 2nd stone and gives it a hit of 2.2 taking it out of the Excellent range and into the Very Good rating and the score decreases as it goes to the 3rd stone with slightly steeper angles and a little bit more leakage.
Before we proceed to the limitations of the HCA, lets review it's strengths as covered thus far.
The question that begs an answer then is "Do the seemingly best FireScope images always translate into the most beautifully cut diamonds?" And ... "Is this leakage which is visible under a reflector (foundation for HCA scoring), discernable to human eyes?" If the answer to any of these questions are "no", then there are some apparent kinks in the system that consumers who consult this tool need to be made aware of as well.
[此贴子已经被作者于2010/7/8 17:50:04编辑过]
Here are the more accurate HCA Results based on actual measurements of Sarin and not rounded with GIA's calculations.
So
one serious limitation of the HCA is that this HCA 2.3/2.1 (depending
on which Report's numbers you input) happens to be as bright as as
stones scoring under 2 (and in some cases even brighter!). Here is a
graphic side by side of this 1.89ct G Si1 alongside a comparably sized
1.84ct F VS2 diamond which scores a 1.1 on the HCA.
1.84ct F VS2 HCA 1.1 | 1.89ct G Si1 HCA 2.3/2.1 |
|
|
![]() |
![]() |
![]() |
![]() |
To view a video of this comparison under diffuse lighting (for brightness) and spot lighting (for fire) click here.
Using the same diamond as an example, let's look at limitation #3 of the HCA.
Measured Proportions | HCA Results | |
GIA Measurements (slightly rounded). | ![]() |
![]() |
The same diamond as measured by the Sarin Machine. | ![]() |
![]() |
The same diamond as measured by an OGI Scanner. | ![]() |
![]() |
![]() |
a. The Table: Average of 4 table measurements. (1 table facet) b. Crown Angles: Average of 8 crown angle measurements. (8 crown main facets) |
![]() |
c. Average of 8 pavilion angle measurements. (8 pavilion main facets)
1 table |
The facets the HCA is not taking into account are
![]() |
d. 16 upper half facets. e. 8 star facets |
![]() |
f. 16 lower half facets.
|
Facets measured |
Variance or difference from minimum to maximum |
crown angles (33.4 - 34.3) |
.9o |
crown height (14.3 - 15.5) |
1.2% |
pavilion angles (40.1 - 41.4) |
1.3o |
pavilion depth (42.2 - 43.3) |
1.1% |
table % (55.2 - 56.5) |
1.3% |
culet off centered |
.6% |
table off centered |
1.0% |
Note this diamond for comparison.
Facets measured |
Variance from min to max |
crown angles (34.2 - 34.4o) |
.2o |
crown height (15.0 - 15.3%) |
.3% |
pavilion angles (40.7 - 40.9°) |
.2o |
pavilion depth (42.6 - 42.9) |
.3% |
table % (56.2 - 56.5&) |
.3% |
culet off centered |
.1% |
table off centered |
.1% |
Note
how tightly the variances are on this diamond when you take into
account minimum/maximum measurements. No comparison between the 2 when
it comes to precision. This diamond is cut to a MUCH tighter level of
precision which the HCA does not tell you. This limitation which can
be examined in the numbers can also directly impact Limitation #7.
HCA 2.3 | HCA 1.1 |
![]() |
![]() |
Dug out diamond (GIA Vg/AGS 1) | Painted girdle diamond (GIA VG/AGS 0) | Classic girdle cutting (GIA Ex/AGS 0) | |
Proportions | ![]() |
![]() |
![]() |
Helium paint/dug results | ![]() |
![]() |
![]() |
HCA Results |
![]() |
![]() |
![]() |
Photography | ![]() |
![]() |
Each of the graphics on the left are showing this diamond (specs above) which is on the right hand side in each of the pictures. A decrease in optics can be seen in both the dug out stone and the painted stone as exhibited by the presence of more dark areas than there are bright reflections of white light. This is yet another instance where a GIA Ex/AGS Ideal with an HCA score >2 is brighter than these stones with HCA scores < 2. |
To view a video of this comparison click here. | To view a video of this comparison click here. |
Proportions via Helium Scan | DiamXray Analysis | HCA Results |
![]() |
![]() Note: The white areas seen in this graphic are the result of inclusions (feathers) and not leakage. |
![]() |
GIA FacetWare Results on this diamond | ||
|
||
AGS Performance Grading Software results on this diamond. |
||
|
你需要登录后才可以编辑
登录 | 注册