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Gem colour grading


#1

G’day, all you experts in the GIA grading system and similar.
Instead of the vague terms like ‘slightly bluish green’ and so on; why
isn’t a simple colourimeter like for instance the old Lovibond system,
or a spectrometer used to convert subjective visual into
figures which would be understood by everybody who’s business it is
to know about gem colouring?

I was (very long ago) employed in routine testing - quality control -
in a plastics factory where on a regular basis I subjected samples of
casein, dyestuffs, pigments, and liquids to various tests, including
colour. Colour differences in the product was important; as a batch
of plastic to be made tomorrow had to perfectly match a batch made 2
years previously (as in buttons, for instance) For this we used a
Lovibond Colourimeter in which a large number of standard colour
filters were available to be inserted or removed until the built up
colour matched the sample as perfectly as possible. This visual
system was superseded later by an electronic system, which took most
of the subjectivity out of the analyses, and in both systems one
finished up with readings like Red: 7.35 Blue: 6.12; and so on. One
could then apply these figures to one’s raw samples, adding exactly
the correct amount of dye or pigment to produce an accurate final
colour. This boiled down to accurate colour analysis of various
samples to enable one to synthesize a final colour.

I am merely suggesting that a similar non-subjective method should be
applied to the colour of Oh yes, I do know about the
pocket spectroSCOPES used in gem testing (I’ve used one) but they
don’t provide a numerical reading as a spectroMETER would. However
these instruments are bulky and expensive. Surely something portable
and battery operated should be available these days? Enlighten me
please. Cheers, –

John Burgess; @John_Burgess2 of Mapua Nelson NZ


#2

I leave it to others to speak for themselves, but from my perspective,
colorimeters are too precise, much in the same way that Pantone
Colors are.

Not to mention that a colorimeter is a major investment, is it not?
The GIA system is several hundred dollars, as I recall (I may be
wrong).

I would want to be able to grade gemstone colors on my own, without
yet another piece of expensive equipment.

Kat


#3

Using a colorimeter works well on opaque colors. It even works well
on transparent colors as long as a consistent background is used.
But when a colorimeter is used on a transparent faceted gemstone, the
brilliance and scintillation causes the results to vary. Also when
judging color on a faceted stone the colorimeter had difficulty in
judging color on light return (the face-up color we perceive). This
light return and concentration of color in not only dependent on the
hue, tone and saturation of the host stone (rough/uncut) but it is
modified by the cut and possibly pleochroism of the stone. A
colorimeter cannot make a conscious judgment between, extinction,
light return or a saturation of color caused by the facets and
movement (scintillation) of the faceted stone. GIA has researched
the use of colorimeter for a while and found that the judgment
ability of a qualified grader worked more consistently than the
accuracy of a machine. If the machine was aimed at the wrong area of
a stone, it would give a color coordinate of a color that doesn’t
face up from the perspective of a human eye.

Plus the cost was to high, especially for a tradition motivated
(cheap) industry.

Note: as machinery (colorimeters) get more sophisticated, they can
pinpoint specific areas of relative color within a gemstone. But
they are still only as good as the user. And in the circumstance of
communicating color from one dealer to another, both parties would
need colorimeters for an accurate comparison. Vague word terms at
least give an idea of color description, and the current systems only
seem vague from an analytical scientists point of view, but may seem
complex for your average gem hustler.

Arthur


#4

Hi Gang,

I don’t know of any battery powered colorimeter, but Howard Rubin of
Gemdialogue Systems has a good, easy to use & transport tool for
defining the color of a stone. The product is called ‘Gem Dialogue’.

It consists of a book about 8 inches wide by 4 inches long & 1 1/4
inch thick. It has transparent pages in various colors, shades & tones
that are used together to recreate the color of the stone in question.
Each page is numbered & the numbers of combination of pages defines
the stones color.

Howard also has another book that translates the Gem Dialogue numbers
into the GIA numbers & 1 or 2 other color id schemes.

I’m not sure of the price of the Gem Dialogue product, but I think it
is around $350.

Howard’s been in the gem business for a long time & has done a lot of
research to develop this product. It looks like a very good tool.

I’ve not used it, but have seen it at the Tucson show. I’m not
connected with Howard or the company.

Dave


#5

Hello John, Before I put up my site I searched for such systems. I
went to last fall’s Hong Kong jewelry show and asked ever provider of
instruments for some analytical system. The closest anyone could
come was the Munscelle system.

Just tell me where it is and not only will I buy one, but offer them
for sale on my site. Remember, the buyer and seller have to have the
same system.

Gemex’s photospecrometer is fantastic; giving accurate numerical
readings for hue, saturation and tone. And all this in diffused and
reflected light. But US$50,000!!!

Enlighten me please.


#6
 Vague word terms at least give an idea of color description, and
the current systems only seem vague from an analytical scientists
point of view, but may seem complex for your average gem hustler. 

I think you’ve put your finger on it here. While there are some very
knowledgeable people in the gem industry, there are many hacks
(“average gem hustlers”) who don’t know much and don’t want to. I
think there might be ways to use a colorimeter and average the input
to it (through frosted glass or diffusion domes?). Marty Haske
evidently has one that works, at least for diamond grading, and, last
I heard, they were programming it for colored stones. The problem
is, people are just resistant to it. If consumers eventually get
savvy and there are more 20/20 stories on it, maybe there will be a
demand for better appraisal. I haven’t ever seen a scientific study
of color perception in grading/appraising, but I would bet you are
really rolling the dice even with a good appraiser when stones of
1000/ct are in question. I think you may send me back to the
psychological literature on perception . . .

Best Regards,
Roy


#7

Mostly it’s cost. GIA, when they first started working on color
grading, did develope exactly such an instrument, the GIA colormaster.
But it’s cost deterred most folks, plus it’s not exactly a small
piece of equipment. Now they’re basing the standards on another more
portable color reference tool, the Gemset cast acrylic simulated
stones.

Another aspect to consider about color grading is that it’s not
actually quite as simple as a spectrographic analysis. The apparent
color of a gem is far more than just a single hue, saturation, and
tone value. It might be, were it not for the fact that most gems are
at least slightly pleochroic, and many are not evenly colored, and
other aspects of the gem, brilliance, windowing, extinction, clarity,
etc, can also affect the visual perception of a gem’s color. while
you can use spectrometer or other instruments to take a precise color
measurement, it turns out that the eye is still a more accurate
instrument when used as a color comparison tool with a reference
standard to compare to, rather than something to just guess at a color
without a reference. The spectrographic analysis generally isn’t as
consistant as a trained human color grader can be, since the human
will be seeing the whole stone, not just one optical path. and the
human will likely be evaluating the stone with varying light and
viewpoints, and taking into account other aspects of a stone that also
affect the visual color perception, such as clarity and cutting.
Since it’s the visual appearance of the color that’s actually
important to it’s value, this turns out to be more suited to our needs
than a precise CIE value measured spectrographically.

It’s important to understand that the somewhat imprecise sounding GIA
(or other) color description terms are not actually as vague as they
sound. Each refers to a precisely defined standard, which can be
referred to in grading by comparing the actual stone to the standard
Gemset color sample. Unlike color chips, the samples are transparent,
and facetted to reflect light in much the way as the colored stones
will, so your eye’s perception of that color is much more closely
matched to the way the stones themselves look.

Now, the first thought that this brings up is that perhaps the color
description tools are then less useful to someone who doesn’t also
have a Gemset sample set handy. True enough. But this is no
different from the similar situation in any other color grading, such
as diamonds. Unless you’ve got the visual standard handy, the verbal
description relies on imperfect human memory of color. No real way
around that. But if the color grades used are assigned to a gem
correctly in the first place, then they become useful even if the
subsequent reader/user of those grades does not have the standard
samples. Price list surveys, for example, may list market data based
on those standard color samples. Two stones equally graded can be
relied upon to be based on the same language. If color grades are
assigned based on just the graders visual impression, without any
reference standards, then the system, any system, becomes a totally
useless white elephant. The GIA gemset system is, despite it’s
admitted weaknesses, still a remarkably good system. I have, and
use, not just that gemset system, but an AGS colorscan set, and the
gemdialogue system. This last I find cumbersome and almost useless,
though some folks like it. Colorscan is very good, but limited to the
few gems it covers. The GIA system sometimes requires you to
interpolate a bit between grades, but this works. And then, for those
without the system handy, the verbal descriptions still give useful
Not as much as with the samples to refer to, but still
enough for most uses. compare that to say, a AGS colorscan grade of
208, one of the emerald samples. Not much good to you if you don’t
have the sample card…

Aw hell. I’m rambling again.

Peter


#8
   As I said in my original e-mail.  GIA has copyrighted their
system and does not distribute their system freely.  If I use it I
would be violating their copyright. 

GIA would be pretty foolish to design and teach a colored stone
grading and color description system if they did not intend to allow
people to use it. You MAY use it without violating their copyright,
just as you can read a book about jewelry making and then make jewelry
with that without violating the books copyright. And you
can describe the colors of gems, in writing or otherwise, using their
terminology without violating their copyright. I’d guess that what
you can’t do is write books about their systems and sell those books
teaching folks to use their system. that’s what THEY are selling,
and your books would likely be violating their rights. But they
publish their system with the stated intent and hope that the industry
will adopt and use it, and of course, buy their coarse to learn about
it in the first place. If the system is not adopted and used by the
industry, then teaching it to their students is a waste of time. What
they are in business to do is to educate. They sell their educational
materials, training, books, classes, etc. The system itself is
available for use by those who wish to use it, especially those who
have taken the course and thus can be relied upon to be actually using
the complete system as it was intended to be used. Proper use of the
terminology does require that one state that one is using the GIA
color description system, and thus due credit is given to GIA for
their system when its used. No system, GIA’s or yours, would be valid
if the system itself were not identified in like manner when it’s
used.

Also, you mentioned that your system reveals saturation and tone
values, implying that GIA’s system does not. That’s not true. The
GIA system does reveal these. A proper GIA color description includes
both the hue notation and the numeric notation for saturation and
tone. An example: slbG/6/4 is the shorthand notation for a “medium
dark, moderately strong, slightly bluish Green”

   Also, although I have never seen a GIA handbook  on color stone
grading, I have never heard mention that GIA uses percentage of
brilliance in grading.  

The GIA colored stone grading system does indeed consider percentages
of not just brilliance, but windowing and extinction as well in
arriving at a cut grade, along with symmetry, finish, and several
other aspects. It’s not specifically stated in the actual final cut
grade, but all those factors are, indeed, taken quite thoroughly into
consideration in arriving at the final cut grade. And they’ve got a
pretty decent (In my view) system for classifying clarity in colored
stones as well. A daunting task when you consider the different
ranges of available clarity in colored gems, and what’s acceptable in
one gem vs. another.

While I’d say you’re to be commended for your attemp to create order
and uniformity in an industry where chaos still seems to reign
supreme, I can’t quite get past the feeling that your attempting to
reinvent the wheel here. The GIA system is really quite complete.
You’re going about setting up your own system without actually
investigating and learning about the already established system that’s
already there, and being taught to many students every year. I wonder
whether the introduction of another competing system will not hinder,
more than help, the goal of consistant color communication in this
industry. I would point out to you that one of the greatest strengths
of the GIA system is that it, like their diamond grading system, is
NOT based just on what they’d like to see in the industry. It’s based
on the evaluations and practices of what the industry already looks
for, but doesn’t consistantly describe. So, for example, your post
states “IMHO, I believe percentage of brilliance…” etc. The thing
it, right or wrong, this is just your opinion. And you then design a
system based on your views. The result cannot belp but be weighted
to just your own perceptions of the way it should be. The GIA
systems, on the other hand, are based on the extensive research of
many people, and the practices found with many dealers and experts in
the field, both at GIA and around the world in the industry. While no
two people will ever agree on everything, the result of developeing a
system in the way GIA did is that it will be a pretty good average of
the way most of the industry believes things need to be done. That’s
more likely to suit the needs of most of the industry than any system
developed by just one person.

David, unless you’re in a position where you really cannot afford the
relatively modest cost (considering that they offer time payments, and
in comparison to the cost of so many other aspects of the jewelry and
gem business) of the GIA colored stone grading correspondence course,
I would strongly suggest that you at least take that one course. I
suspect that once you bite that bullet and take that course, you’ll
find yourself wondering why you ever resisted it. It’s really very
well done, and far more complete than you appear to realize. And it’s
not a difficult course to complete. Without completely familiarizing
yourself with your competition, how can you properly assure yourself
that a new system is warrented, or that you’ve indeed done it the way
it should be done?

There are, indeed, some weaknesses in the GIA system. Most notably,
within some gem types, the differences in value between really subtle
differences in color can be quite significant. I’ve yet to see any
system based on strictly visual estimates of hue, color, or
saturation, that can really differentiate. The GIA gemset based
standards come close, but not quite perfectly, when trying to describe
the colors of the most costly of gems. Emerald, in particular, seems
to fall into this catagory, and some rubies too. The GIA colormaster
is a bit better on these subtle gradations, but falls short in the
most intense saturations. Of all the systems I’ve seen, The AGS
colorscan system, which is no longer offered, came closest within
these most costly gems, yet it had major problems with anything other
than the gems for which it offered actual color chip samples (Not to
mention major problems just with delivery and backorders of the system
samples themselves from AGS, a long dead story that still makes me
angry as I recall it, so I won’t). So it was quite limited in it’s
own way. The GIA system’s weaknesses come simply from the fact that
it’s been designed to describe the color of any gem in any color.
That’s simply too wide a field to be done without a few weak areas.
But they come pretty damn close, better I think, than anyone else has
done. At least in my view…

Well, enough of this soapbox. I think I’m starting to sound like a
preacher. Sorry 'bout that…

Peter Rowe


#9

Peter: Would be interested in hearing why you find the Gemdialog
system troublesome.
Roy


#10

In another forum on another site I shudder upon hearing people in
"the industry" debate what should and should not be revealed to the
public. With the exception of pegasus and laser drilling the diamond
industry looks lily white compared to the colored stone business. I
wince every time I hear “created” and “cultured” colored stones.

I’m for absolute full disclosure. Tell a customer everything and let
him decided. I seldom have to look over my shoulder. (except for
Bangkok drivers);>)


#11
   Mostly it's cost.  GIA, when they first started working on color
grading, did develope exactly such an instrument, the GIA
colormaster. But it's cost deterred most folks, plus it's not
exactly a small piece of equipment. 

It’s always cost. AFAIK the colormaster is nor longer available.

Now they're basing the standards on another more portable color
reference tool, the Gemset cast acrylic simulated stones.  

Which is also quite dear. I went to GIA in Bangkok and was offered
the set for US$650.

I work with two colors, red and blue. I need a limited number of the
standards. They refused to sell me a partial set. Although I have
heard they sell individual standards to GIA grads at US$2 each.

The first question I usually get about a blue sapphire is, “Does it
have any green in it?”

The first question I usually get about a ruby is, “Does it have any
brown in it?”


#12

Actually I’m quite confident in my ability to grade colored gems.
But I am trying to give my potential customers a better way of
relating to the gems I’m offering. And yes, I know photography
helps, amd micrscopic photography even more.

But if I put GIA’s system on my site I have published it and have
therefore violated the copyright. Linking to a GIA site with the
system would be great . . .but I don’t think one exists.

 But they publish their system with the stated intent and hope that
the industry will adopt and use it, and of course, buy their coarse
to learn about it in the first place. 

Where is it published? I have never seen it offered except as a
course.

 If the system is not adopted and used by the industry, then
teaching it to their students is a waste of time.  

I only partly agree with this. GIA is not the only school where
color grading is taught.

Also, you mentioned that your system reveals saturation and tone
values, implying that GIA’s system does not. That’s not true. The
GIA system does reveal these. A proper GIA color description includes
both the hue notation and the numeric notation for saturation and
tone. An example: slbG/6/4 is the shorthand notation for a “medium
dark, moderately strong, slightly bluish Green”

I think I said I didn’t know since I had never been able to find the
system intact.

The GIA colored stone grading system does indeed consider percentages
of not just brilliance, but windowing and extinction as well in
arriving at a cut grade, along with symmetry, finish, and several
other aspects.

Again, my comment of heresay, this is all I have to go on.

GIA is not the only institute teaching color stone grading.

Their materials are also extraordinarily expensive, IMO.

I wonder whether the introduction of another competing system will
not hinder, more than help, the goal of consistant color
communication in this industry. 

I rather doubt that my system competes with GIA. Standardization
often occurs as a result of numerous systems/standards in many
industries.

Sorry Peter, you’re taking that out of context. I said I believe
brilliance is second to color in a gem.

 And you then design a system based on your views. 

No, I’m designing a system that is based on the primary factors
universally accepted as important in colord

Do I not deserve to have an opinion aside, as well? :>)

The result cannot belp but be weighted to just your own perceptions
of the way it should be.  The GIA systems, on the other hand, are
based on the extensive research of many people, and the practices
found with many dealers and experts in the field, both at GIA and
around the world in the industry.  While no two people will ever
agree on everything, the result of developeing a system in the way
GIA did is that it will be a pretty good average of the way most of
the industry believes things need to be done.  That's more likely to
suit the needs of most of the industry than any system developed by
just one person. 

Again, you’re using the one person against the world issue. The
system I have put on my site is based much on GIA, much on AIGS and
much on my albeit limited experience grading colored gems.

It seem s a bit ironic that some posters on this subject seem to
think my system is nothing more than a cheap copy of GIA’s whereas
you seem to feel it’s little more than an opinion.

I understand your loyalty to GIA. Would you still feel comfortable
making the above statement knowing I had studied (residence) “Colored
Stone Grading and Appraisal” at AIGS in Bangkok?

Regards,
Dave Webster
www.asia-gems.com


#13
   Peter: Would be interested in hearing why you find the Gemdialog
system troublesome. 

Not so much “troublesome”, as more cumbersome, and for want of a
better word, less elegant as a solution. While it’s possible to get a
color match for many colors out there, I just find their system of
multiple overlaid transparent filters to be awkward and slow to use.
Plus, In my opinion, it’s harder to match the colors since the
gemdialog system is using simple flat transmission filters. A gem
shows you it’s colors in several distinct types of visual field, the
areas of brilliance, ie. total internal reflection from the pavilion
of light that entered through the crown, full extinction (just dark
areas lacking of any reflection), and windowing, where you see though
it, but no brilliance. And the brilliant areas will vary from slight
reflections to intense ones. Within that range you then must decide on
which areas are the most characteristic of the color of the whole
stone. It’s a complex visual decision. And the difference between
the overall appearance of a gem versus the color seen in a plain
transparent transmission filter is considerable, even when the colors
match. You have to concentrate on it just to be sure you’re correctly
seeing the color of the filter before you can decide whether it’s a
match for the stone. Sure, it’s doable, and not actually all that
difficult. But the GIA gemset samples much more closely match the
appearance of a facetted stone, and you then can dispense with that
need to convert the appearance of the filter with the appearance of a
cut stone. Makes color matching a lot faster and you end up being
more sure of your calls. But that’s just the way I feel. I do also
know that there are others out there who like and use the Gemdialoge
system. It does offer the distinct advantages of being both cheaper
and closer to pocket size. I personally just prefer either the
gemsets or the no longer available AGL colorscan cards, which are also
highly portable and also give a good visual duplication of the effect
of a colored stone. In diamond grading, until the advent of good
colorimeters, the only really good means of color grading diamonds
were sample stones. And diamond masters are better and more accurate
than C.Z. masters. A similar situation exists for colored stones.
The best system would be master sets of all signifiant color grades of
each precious stone, to which one could compare and assign consistant
grades when you had a stone of a similar type to your masters.
Obviously, this is not a really workable worldwide system for colored
stones, despite the fact that it’s exactly what’s been done for
diamonds. The gemset, though, comes reasonably close to just such a
set of master color grading stones for colored stones. My gem
dialogue kit just sits unused in a drawer except when I need to refer
to someone elses grading done using that system.

Peter Rowe