Can cut influence colour-grading?

Would be very interested to read your thoughts Paul?


Some starting observations, for two stones w/ the same girdle thickness and culet size
-if evaluate top-down (looking directly into the culet, line of sight perpendicular to the table plane) a shallower stone will look comparatively less tinted
-if evaluate through the girdle (perpendicular to the table plane) a shallower stone that is distributing its volume in spread will look comparatively more tinted
-if evaluate from normal to the pavilion (exactly 45d), the proportions will still dictate whether there are more or fewer layers of diamond in your direct line of sight - a quickie using these two stones both 1.01ct http://www.bluenile.com/diamond-search?filter_id=0&track=head#diamonds_pid=LD01892833,, http://www.bluenile.com/diamond-search?reason=tryinteractive#certViewer_cert_pid=LD01374120|cert_num=1 and assuming you are looking at the middle of the pavilion
L - 64T, 31.5C, 40P, thinGirdle, 6.7mm diametre
R - 55T, 35C, 41.8P, th-slthGirdle, 6.36mm diametre



But, questions that I think need trade experience to answer
-Whether the eye could actually distinguish that relative diff in saturation b/c of the extra thickness of tinted material
-How well graders can denormalize what they are seeing and grade "as if a chip is taken off the stone"
-If/how/how much age (eye lens yellowing) is significant

Thank you, Yssie, for the reply.

You are understanding my thoughts, but I do not think that you should look simply at material thickness. A more detailed study is necessary to check where the light is originating from, basically reverse-ray-tracing, with a completely different location of the observer and how long these rays are traveling within the diamond.

Intuitively, I think that it is possible to 'maximize' cut in order to obtain the highest colour-grade in a lab. This means that what we often describe as 'true body colour' in reality is something different.

And from experience, I know that colour-grading of various fancy shapes is often difficult, because of them showing various tints in various parts of the stone (in a colour-grading-tray). Where does a lab assess the 'true body colour'?

Live long,

I'll definitely be watching this discussion


I do have a newbie question - I've been thinking that colour intensity grading (like G vs J, or light vs vivid) is primarily one of saturation, not hue - is this a good starting assumption?


The issue with a path length analysis from what I can tell is that there's no general solution to anything - whatever results you get are valid only for that stone in that configuration viewed from that angle - so even with hundreds of these, say, what does it all mean? What is the goal - beyond max and min? Could you elaborate on this?

Or.. just grade the rough colour and avoid all those pesky facet problems.. hey, it's 4am out here, I had to say somethin'..

Written by Yssie » December 17th, 2010, 6:07 am:Or.. just grade the rough colour and avoid all those pesky facet problems.. hey, it's 4am out here, I had to say somethin'..

Even that will be limited by the size of the stone, large stone, longer path.

Just wondering if there will be a discussion, Yssie. We will need to await that.

If your last question is about my goals with this question, it definitely is not to challenge the current colour-grading-method in order to have it replaced by something else. It is more about understanding the effect of certain factors.

At the same time, if I am right, it undermines the currently prevalent notion that by judging colour in its current methodology, one observes the true body-colour of a diamond. This might be a half-truth, upon which many are basing their 'expertise'.

It could well be that a lot of other 'expertise' is also based upon similar 'half-truths'. Maybe, this will open up certain discussions.

Finally, in this specific topic, if we get some confirmation about my theory, it opens up possibilities of maximizing graded colour from the same rough. In fancy colours, this is already a specialty, because colour is graded face-up. But can the same be done for colour-grading of D-Q-stones, considering the different position of the observer?

Live long,

Written by Paul-Antwerp » December 17th, 2010, 7:26 am:Just wondering if there will be a discussion, Yssie. We will need to await that.

If your last question is about my goals with this question, it definitely is not to challenge the current colour-grading-method in order to have it replaced by something else. It is more about understanding the effect of certain factors.

At the same time, if I am right, it undermines the currently prevalent notion that by judging colour in its current methodology, one observes the true body-colour of a diamond. This might be a half-truth, upon which many are basing their 'expertise'.

It could well be that a lot of other 'expertise' is also based upon similar 'half-truths'. Maybe, this will open up certain discussions.

Finally, in this specific topic, if we get some confirmation about my theory, it opens up possibilities of maximizing graded colour from the same rough. In fancy colours, this is already a specialty, because colour is graded face-up. But can the same be done for colour-grading of D-Q-stones, considering the different position of the observer?

Live long,

Oh I didn't mean anything like you being a wayward challenger Paul I'm just wondering how you would go about using the information you would get from an experiment like that - what you would expect it to tell you, other than longest and shortest path length POVs. My friend has a loose larger J pear w/ notation of brown and I can certainly see what you mean re. differences in colour easily enough.. I too hope a good discussion begins.

Written by Paul-Antwerp » December 16th, 2010, 8:55 am:We probably all agree that certain levels of cut-quality improve the observed colour face-up. Just to be clear, this is not what my question is about.

Colour-grading is done from the side, because theory says that is how one can observe the true body-colour of a diamond. Is this correct, however? No, diamond size and shape can make a huge difference. the sometimes discussed problem of grading 5ct diamonds with 1/2ct masters will not match lab grading, rotating a long EC would give several different color grades depending on which axis it was graded

Two sub-questions to that:

1. Can one design and cut diamonds in such a way that they would appear less tinted when their colour is graded? yes I think so but haven't worked on it so no proof
2. If so, are certain shapes better at hiding their colour? yes I think so, but haven't worked on it

Live long,

I'm inclined to think so since fancy colors are often cut into radiants and other busy cuts to pick up more color for a more intense grade.

My own experience with it is not very scientific but I purchased a 2.23 estate european cut diamond the seller described as M VS2 in a platinum setting. I sent it to Ari Madillian for a minor re-cut and repolish and he said the color prior to re-cutting was more like Q-R but I should not worry because I got it at an excellent price. Once the diamond had a home in a new platinum setting after being reduced to 2.06 I took it to a well respected appraiser who ultimately decided to call it L VS2 after a lot of consideration but noted it was possible the platinum was helping the color. My diamond started out with a frosted girdle and I had it faceted. I have heard conflicting things on whether frosted girdles help or hurt the color but mine certainly appeared far whiter once the girdle was faceted. Since none of the grading was done by a lab I'd say it's pretty subjective but its interesting to me that experienced peoples' color perceptions could vary so widely. With antique cuts, my take has become, "You either like it for what it is, or you don't".

question...how does size influence color? I have read that larger diamonds will show more color, but is a 0.40 carat I the same color as a 4 carat I, just the 4 carat looks more tinted? How is this taken into account when grading?

Actually, newbie question, color is graded by person, right, comparing to master stones? I have seen some GOG videos where they have a color machine, but those are not used? Do graders have different master stones for the different cuts and sizes?

From the same rough crystal, the smaller stone routinely gets a better colour (one or two grades) than the bigger stone. Length of ray-paths probably is the major cause for that, given that in our case, the cut is (about) the same.

In the same-size-stone however, I think that cut can influence colour-grading.

Live long,

Written by Paul-Antwerp » December 16th, 2010, 8:55 am:We probably all agree that certain levels of cut-quality improve the observed colour face-up. Just to be clear, this is not what my question is about.

Colour-grading is done from the side, because theory says that is how one can observe the true body-colour of a diamond. Is this correct, however?

Two sub-questions to that:

1. Can one design and cut diamonds in such a way that they would appear less tinted when their colour is graded?
2. If so, are certain shapes better at hiding their colour?

Live long,

I believe shape and girdle finish can alter the profilic color appearance of the standard cuts (eg. Crown + girdle + pavilion type cut).
But if we are bringing zoning into the equation..., there are IMO many more issues that must be taken into consideration.

Hi paul,
Its bed time, and have not read all yet - but this old image might help.
The side graded color is also affected by the ray path length - look especially at the center of the emerald cut for example.

Attachments

Color grading face up and side4.JPG (185.58 KiB) Viewed 760 times