Tone and saturation

[zeolite]

On another thread, there was a mention of the relationship between tone and saturation. I gave an explanation, and it was probably too brief. Here is an attempt to expound on that. You are welcome to disagree.

In general, there is a correlation between increasing tone and increasing color saturation. When a sample of a gem is darker in tone, it usually has more color saturation.

I can think of two situations where increasing tone does not result in increasing saturation. That is the case of gems with a gray or brown modifier.

For the moment, I’m going to side step that, and discuss a situation when gems have equal tone, and equal hue, but one has significantly higher saturation.

Before I do that, let me attempt to define 3 attributes of color: tone, hue and saturation. Tone is how light or dark the gem is, about how much light it absorbs. Hue is the single wavelength that represents the color (there is an exception to this). For example, a ruby can be orangy red, red or purplish red. Let us say red is the hue. Saturation - I think everyone understands this – the intensity of the color.

It is quite possible for two gems to have equal tone and equal hue, but significantly different saturation. This occurs when the chromophore causing the color is different.

http://www.allaboutgemstones.com/gemstone_color.html

It is possible find a red garnet (colored by iron) with equal tone and equal hue to a chrome pyrope, colored by chromium. Yet the chrome pyrope will have a more saturated color.

It is possible find a pinkish-orange malaya garnet with equal tone and equal hue to an imperial topaz. Yet place them side by side, and the topaz shows more intense saturation.

It is possible find a green tourmaline (colored by iron) with equal tone and equal hue to a chrome tourmaline. Yet place them side by side, and the chrome tourmaline shows more intense saturation.

It is possible find a pink sappphire with equal tone and equal hue to an intense flame spinel, both colored by chromium. Yet place them side by side, and the spinel shows more intense saturation.

It is possible find a tsavorite (colored by vanadium or chromium) with equal tone and equal hue to a emerald (colored by chromium). Yet place them side by side, and the emerald shows more intense saturation.

As a super extreme, it is possible find an intense medium blue Afghani tourmaline with equal tone and equal hue to a real Paraiba blue tourmaline (found in Brazil). Yet place them side by side, and the Paraiba totally blows it away in saturation.

My point here, in perhaps less than 10% of all colored stones, is that it is very possible to have equal tone and equal hue, and yet very much different saturation.

 

[T L]

Mr. Zeolite, when you say equal hue, do you mean both stones have the same hue? If that's the case, I think it can be more than 10% of stones. This is how I read your explantion. There are 31 hues of color, let's just use Orange as an example, If we plot this on a three dimensional axis, where x is hue (Orange), y is tone (example: medium tone), can't there be more than one z coordinate to represent saturation? Or is this quite a bit more complex than that? Now if we're just talking orange, that cuts down the sample, but in 31 various hues, I would suspect there to be a greater percentage of stones that can attain deeper levels of saturation, even if they are the same hue and tone

Thank you for the excellent write up. I hope this ends up in a lively, fun and interesting debate.

 

[zeolite]

Yes, both stones have the same hue.

The 3 dimensional model is a man made contruct, an attempt to model what we see in nature. I think it is a lot more complicated than that.

In my write-up above, that is a distillation of viewing, really, millions of gemstones over 25 years of Tucson shows. Also, it is a result of owning too many gems and being able to select tone and hue from a large number of gems and placing them side by side. I only discovered the difference between a Cutting Edge winner malaya garnet being blown away by an imperial topaz, by placing them side by side.

It is also a manifestation of why certain gems are so highly valued. The usual tradeoff is giving up brilliance, going darker in tone, to obtain intense color (saturation). Paraiba tourmalines, flame spinels, Columbian emeralds and Kashmir sapphires break down this distinction. They offer intense color AND at the same time, brilliance.

 

[T L]

That's an interesting pattern in nature if only around 10% of stones can have the same tone, hue, but different saturation. Did you ever write a paper on the subject? I think it would be interesting, especially in regards to gemological science.

Do you find color to be more subjective or precise based on what one trained individual regards as hue, tone and saturation? Do you feel that the GIA is lacking levels of saturation, hue or tone that causes potential subjectivity, or do you think that would be overkill?

Do you feel that some recent finds, like Paraiba tourmaline, have redefined the saturation color scale for some hues?

 

[ma re]

A very interesting thread indeed! But Mr. Z, you probably know by now that 90% (if not more) of Pricescopers prefers...graphic explanations, much more than words

And since you have an admirable collections of gems...

OK, I understand if you are too busy to play around right now, but it would sure be interesting to see examples instead of just a wordly debate.

I''m sure you saw this coming at some point

 

[chrono]

While I have nothing to add at the moment, this thread is a very interesting read and brings up good points about how we view colour and judge gems, and that it is a process that continually evolves.

 

[chrono]

Zeolite,
For the sake of simplifying my questions, I am going to use the same example criteria as you’ve presented, meaning that the hue and tone are equal with only the perceived saturation being the variable. To keep things even easier, I will only compare stones with the same chromophore as examples.

1. Pink sapphire and pink spinel, with the equal tone and hue. Both are coloured by chromium. You’ve stated that when placed side by side, the spinel will show more intense saturation. Are you saying that the very finest spinel will always show better saturation than the very best pink sapphire? If so, does that mean that by the GIA colour scale (used for the sake of fair comparison), the very best pink spinel can be a 4/6 but the very best pink sapphire will never achieve a 4/6 but at best a 4/5? Please clarify if I have misunderstood your statement.

2. The same question applies to a tsavorite and emerald of equal tone and hue. Both are coloured by chromium. However, is the reason the emerald might show more saturation because of the jardin within the stone? Also, doesn’t brilliance mask the richness of colour to a certain degree?

 

[T L]

Chrono,
I think you bring up a good point Is this saturation/tone scale only applicable within a certain gem species? Richard Hughes said that tsavorite will never look as fine in color as the best emerald (see link). I did not say that, it''s just a quote, but assume it''s true for the sake of argument, does that mean that the finest tsavorite can only be a level 5 in saturation, never a 6?

http://www.palagems.com/tsavorite_bancroft.htm

Considering the most coveted of warm hues:
If Burma ruby is the pinnacle of red color as the trade indicates, than all other red gems can only hope to be maybe a 5 in saturation?

You see, for me, I''m very picky when it comes to saturation levels when I personally judge a gem''s color, and this is because I compare it with gems that are truly vivid that I have seen IRL. Am I incorrect in doing so?

 

[ma re]

Another Zeo-like example; apatite and cuprian. Can have the same hue and tone, but saturation makes such a difference. Eventhough some apatites look beautiful, if you''d place them to fine cuprians, they probably wouldn''t look like much.

 

[zeolite]

TL: Did you ever write a paper on the subject? I think it would be interesting, especially in regards to gemological science. No, I think what GIA researchers have published in Gems & Gemology, is vastly better than anything I could write. Anyone truly interested in gems and color, should buy this publication (4 times per year). I have all but 3 issues published in the last 27 years. It is a wealth of ACCURATE scientific knowledge, and superb color photography and color reproduction.

TL: Do you find color to be more subjective or precise based on what one trained individual regards as hue, tone and saturation? I think it is vastly more complex, because the color and especially the saturation, is generated by a different physical/optical method.

TL: Do you feel that the GIA is lacking levels of saturation, hue or tone that causes potential subjectivity, or do you think that would be overkill? I think GIA is taking on a complex subject and doing better with it than I can do. But trying to cram 11 levels of tone and 6 levels of saturation, and making it apply to all colored stones, is not realistic. The saturation of a blue Pariba tourmaline can’t be compared to an aquamarine.

TL: I think you bring up a good point Is this saturation/tone scale only applicable within a certain gem species? Yes, I think it is only applicable to one species, because the means of creating that color is different.

TL: Richard Hughes said that tsavorite will never look as fine in color as the best emerald (see link). I completely agree. As an aside, I don’t own an emerald and never will, because I can’t afford to buy the quality that I like. I don’t think I’ve seen an emerald that I could really get excited about, at less than $150,000. Even if you’ve seen the finest emeralds at AGTA in Tucson, they are low quality, compared to old emeralds, mined long ago, and sold presently at Sotheby and Christie auctions.

I do have a superb example of a tsavorite, but the color is not the same. Demantoids always have noticeable yellow in their green, tsavorites approach pure green, but top emeralds show a strong blue component in their green, along with astounding saturation.

TL: I did not say that, it''s just a quote, but assume it''s true for the sake of argument, does that mean that the finest tsavorite can only be a level 5 in saturation, never a 6? I think you have much too much faith in the GIA system and levels of saturation. They are only a model, to try to classify what we see in nature.

ma re: you probably know by now that 90% (if not more) of Pricescopers prefers...graphic explanations, much more than words And since you have an admirable collections of gems...
OK, I understand if you are too busy to play around right now, but it would sure be interesting to see examples instead of just a wordly debate.

I started to do that, and just gave up in frustration. I am skilled in Photoshop, and can adjust the tone and hue, to match my picture to my gem. But trying to show the saturation is an exercise in futility. On your computer screen, your are seeing colors though color filters (LCD monitor), or phosphors excited by an electron beam (cathode ray tube monitor). Yet the color being generated by photons of light, bouncing off chromium atoms within the emerald crystal structure, is more saturated than these two display technologies can provide. You need to come to my bank vault (after a careful frisking), and view them in person.

 

[zeolite]

Date: 4/15/2010 1:07:03 PM
Author: ma re
Another Zeo-like example; apatite and cuprian. Can have the same hue and tone, but saturation makes such a difference. Eventhough some apatites look beautiful, if you''d place them to fine cuprians, they probably wouldn''t look like much.

I have a superb blue apatite. It was a Cutting Edge winner, and made the front cover of an Asian Gem magazine (Asia Precious?, not sure).

While it is not the color of a cuprian tourmaline (it is deeper toned and slightly greenish-blue, it can stand up in saturation to the top Paraiba blues.

 

[T L]

Thank you for concisely answering my questions, I appreciate it very much. I particularly agree with this comment from you below, and this is were I think this creates way too much subjectivity. It's too bad that the human eye is the best intepreter of color we know, because color is so subjective and comple. This leads to so much unintentional inconsistancy, as well as potential fraudulent grading.

I think GIA is taking on a complex subject and doing better with it than I can do. But trying to cram 11 levels of tone and 6 levels of saturation, and making it apply to all colored stones, is not realistic.

 

[T L]

Mr Zeolite,
If you have time, can you please kindly expand on this comment below? As a physicist, I would love to hear what you have to say about it. I study some physics on the side, but I am not a physicist. My background in more in mathematics.

color and especially the saturation, is generated by a different physical/optical method.

 

[zeolite]

Date: 4/15/2010 3:26:32 PM
Author: tourmaline_lover
Mr Zeolite,
If you have time, can you please kindly expand on this comment below? As a physicist, I would love to hear what you have to say about it. I study some physics on the side, but I am not a physicist. My background in more in mathematics.

color and especially the saturation, is generated by a different physical/optical method.

TL, you probably think I am ignoring this question, or have forgotten about it. I haven''t. The question is simple, the answer is incredibly complex. I also wonder if anyone other than you, Chrono, or LD is even interested or is listening. If anyone else cares, please speak up.

Gems & Gemology discussed this in incredible scientific detail in Fall 1987, Spring 1988 and Summer 1988, covering no less than 50 pages. The read would be challenging for anyone with less than three degrees: physics, geology, and gemology.

I''m trying to condense it to a level that is understandable.

 

[klewis]

Date: 4/17/2010 1:31:46 AM
Author: zeolite

Date: 4/15/2010 3:26:32 PM

Author: tourmaline_lover

Mr Zeolite,

If you have time, can you please kindly expand on this comment below? As a physicist, I would love to hear what you have to say about it. I study some physics on the side, but I am not a physicist. My background in more in mathematics.


color and especially the saturation, is generated by a different physical/optical method.

TL, you probably think I am ignoring this question, or have forgotten about it. I haven''t. The question is simple, the answer is incredibly complex. I also wonder if anyone other than you, Chrono, or LD is even interested or is listening. If anyone else cares, please speak up.


Gems & Gemology discussed this in incredible scientific detail in Fall 1987, Spring 1988 and Summer 1988, covering no less than 50 pages. The read would be challenging for anyone with less than three degrees: physics, geology, and gemology.


I''m trying to condense it to a level that is understandable.

I don''t have the knowledge to contribute but I am very much interested and I''m sure there are many others on PS that are too. Threads like this and others such as CMW''s thread about colour and blue spinel and the recent padparadscha thread are the most interesting to me. So please keep it coming!

 

[T L]

Date: 4/17/2010 1:31:46 AM
Author: zeolite

Date: 4/15/2010 3:26:32 PM

Author: tourmaline_lover

Mr Zeolite,

If you have time, can you please kindly expand on this comment below? As a physicist, I would love to hear what you have to say about it. I study some physics on the side, but I am not a physicist. My background in more in mathematics.


color and especially the saturation, is generated by a different physical/optical method.

TL, you probably think I am ignoring this question, or have forgotten about it. I haven''t. The question is simple, the answer is incredibly complex. I also wonder if anyone other than you, Chrono, or LD is even interested or is listening. If anyone else cares, please speak up.


Gems & Gemology discussed this in incredible scientific detail in Fall 1987, Spring 1988 and Summer 1988, covering no less than 50 pages. The read would be challenging for anyone with less than three degrees: physics, geology, and gemology.


I''m trying to condense it to a level that is understandable.

Mr. Zeolite,
I think I know you very well from your posts, and I had no doubt you were probably working on a write up. I just feel bad because in hindsight, I knew this was probably a very complex question. I feel bad making you feel as if you have to do a significant write up. Although I would love to hear what you say, don''t feel obligated as I''m sure you''re a very busy person.

 

[Sagebrush]

Zeolite,

Your explanation is basically incorrect because it begins with a false premise.

You state: "In general, there is a correlation between increasing tone and increasing color saturation. When a sample of a gem is darker in tone, it usually has more color saturation."

How can this be true? Think of tone as the adding of black or white to a given hue. 0% tone is colorless (there is nothing there) and 100% tone is total black. Hues reach their maximum saturation at certain tonal levels and that tonal level varies with each hue. These are called "saturation gamut limits" (SGL). You can see a graph in Secrets Of The Gem Trade, page 22. This concept is also discussed in more detail by Hofer, Collecting and Classifying Coloured Diamonds, p.167-173 SGL are observable and demonstrably in color science. Only caveat is that color science and gemology reverse the scale. That is what we call 100% tone a color scientist would reverse the scale and call total black, 0% tone.

Take a bucket of sky blue paint, add a bit of black and you get a darker say evening sky blue keep adding black and eventually you reach...black. The hue blue reaches its SGL at about 80% tone beyond that saturation falls off. There is a correlation with gemstones, if you think about it, the finest sapphire is also 80% tone. So, the brighter the better! Sapphires at 85% we call overcolor or over dark. This is why I often tell clients that a fine sapphire teeters on the brink of being too dark.

there are eight chromatic hues but they are each unique and the relationship between hue and quality/value in gemstones must be approached one hue at a time. Yellow, a big bold hue, reaches its SGT at a much lighter tone, about 20%, orange 25% and so forth. Again there is a direct correlation with gemstone quality though it is fair to say that the market favors a richer hue to a brighter and thus prefers oranges and yellows that are tonally a bit beyond the SGL, slightly and richer in tone.

 

[zeolite]

Hi Richard,
Welcome aboard. We’ll have a lively discussion:

When I mentioned “When a sample of a gem is darker in tone, it usually has more color saturation”, I certainly was not thinking of all the way to black. I am quite aware that there are upper limits of dark tone, where that does not hold true. Examples are overly dark blue sapphires, dark chrome tourmalines, and dark red garnets. I thought that was so painfully obvious that I didn’t even mention it. My mistake. I was thinking of common gems is the 20-75% tonal range.

This link, look under saturation,

http://gemologyproject.com/wiki/index.php?title=Color_grading

which you probably wrote, shows that maximum level 6 saturation occurs in medium dark toned hues. That is the upper (darker) limit of tonality that I was considering.

As you wrote “Hues reach their maximum saturation at certain tonal levels and that tonal level varies with each hue.” Blue sapphire, and chrome tourmaline reach that point at a higher tonal than ruby or emerald. You mentioned that point is lowest for yellow. Here’s why:

http://en.wikipedia.org/wiki/Gamut

The MacAdam limit, on which the most saturated (or "optimal") colors reside, shows that colors that are near monochromatic colors can only be achieved at very low luminance levels, except for yellows, because a mixture of the wavelengths from the long straight-line portion of the spectral locus between green and red will combine to make a color very close to a monochromatic yellow.

I also mentioned that gems with a gray mask will go darker in tone, but the saturation does not improve at all. This is because the gray mask is attenuating all colors relatively equally, which kills the brightness without adding anything in return. On the other hand, rubies pass some light in the violet-blue region (where the eye is not very sensitive), block strongly in the green, and transmit very much in the red. The result is an intense strong red. The gems that are likely to show a gray mask, are some aquamarine, many blue spinel, and all iolite (cordierite).

Gems & Gemology Fall 1987, page 131:
In general, colors produced by iron, will be quite different from those generated by chromium in the same mineral; in spinel Fe2+ causes a grayish blue color, while Cr3+ causes red.
Gems & Gemology Spring 1988, page 7:
(discussing iolite) Iron ions substitute for Al or Mg atoms, so the intervalence charge transfer will occur in the 3 directions in which the Mg and Al atoms are aligned. One of these directions is strictly parallel to the a-axis, so light vibrating in this direction will be strongly absorbed, producing a very intense blue. There is no IVCT parallel to the c-axis, so no charge transfer for light vibrating in that direction is observed; the light yellow color is due to isolated Fe3+
(my comment) The strong gray color in iolite is in this c-axis direction. It is interesting that isolated Fe3+ produces gray, yet Fe in IVCT (intervalence charge transfer) direction, produces strong color and no gray.

 

[deorwine]

This is really fascinating!

zeolite, for stones with an identical (near) pure blue hue and identical tone, which would have the greatest saturation? Sapphire, spinel, tourmaline, benitoite, iolite...something else? My guess is spinel, from synthetics I've seen, but I wouldn't be surprised if that were wrong.

 

[T L]

Date: 4/17/2010 10:32:02 PM
Author: deorwine
This is really fascinating!

zeolite, for stones with an identical (near) pure blue hue and identical tone, which would have the greatest saturation? Sapphire, spinel, tourmaline, benitoite, iolite...something else? My guess is spinel, from synthetics I''ve seen, but I wouldn''t be surprised if that were wrong.

Thank you Mr. Zeolite for pointing out such amazing scientific information as always. It''s fascinating to know that all the color we see are really caused by wavelengths (in simplistic terms). Deorwine, if I may, but I think it depends on the tone. For medium tone, the most saturated blue stone I have ever seen was a blue spinel and hauyne (we''re talking pure blue) and I suspect in medium dark to dark tone, it would be very fine sapphire (thnk Kashmir). I don''t think blue tourmaline, nor benitoite and definitely not iolite can compare from what I''ve seen (I could be wrong). Paraiba is a slightly bluish green or greenish blue, so not really a pure blue hue, so I don''t think that one counts.

 

[zeolite]

Deorwine,

Iolites have a large amount of gray, and all of the benitoites I’ve seen have noticeable gray. Synthetic cobalt spinels have an intense color, though it is purplish blue, and well – synthetic, so maybe it doesn’t count. Hauyne has very strong blue, and Paraiba blues are also quite intense, though usually lighter in tone.

Thank you TL, for your kind words. I have a symphony performance tomorrow, and then off to the other side of the planet for awhile. While I could find an internet connection there, I don’t plan to.

 

[T L]

I should have clarified about blue spinels. The ones I was referring to with vivid blue saturation are the Luc Yen ones from Vietnam, and they remind me of vivid blue hauyne. They are extremely rare and costly though.

Mr. Zeolite,
Have a nice time at your symphony performance and your travels.

 

[Barrett]

Have fun Zeo..hope you ain''t fllyin'' to Nothern Europe

 

[Sagebrush]

Zeolite,

Great post, a philosopher taking on a physicist I should have known better. I love that wiki link. As to questions of which green stone is more saturated, that leads to generalizations and all generalizations are false and that puts us on the road that most neophytes take, buying the brand rather than the stone and most of the members of this forum are beyond that. That is why Kashmir aquarium gravel sells for such a high price. We can certainly talk about the tendency of spinel to show a grayish mask, but a great one won''t and that is what separates the wheat from the chaff.

I have to take issue with my friend Hughes. He likes emerald, so do I but the green in emerald is not necessarily more highly saturated than tsavorite. It is a combination of factors, the small c''s of Color: hue, saturation, tone and in the case of emerald, Crystal and lets not forget UV fluorescence. Looking deep into an emerald crystal reminds me of the jungle foliage surrounding the mines. The color has life, the quality of looking into a deep green jungle pool. In very fine emerald the their is a honey-like transparency. Tsavorite rarely displays the transparency, usually lacks the blue secondary hue and when cut, the superior refraction produces a crisp, no nonsense sort of brilliance that contrasts with satiny quality of emerald.

For those interested in the aesthetics of emerald, I recommend Ronald Ringsrud''s new book, Emerald, A Passionate Guide. I reviewed it in my blog, Gemwise.

 

[T L]

Date: 4/18/2010 7:57:52 AM
Author: Richard W. Wise
Zeolite,

For those interested in the aesthetics of emerald, I recommend Ronald Ringsrud''s new book, Emerald, A Passionate Guide. I reviewed it in my blog, Gemwise.

I second that recommendation on that book. It''s glorious. One of my favorite gem books, and I have many. I also have a few books (including this one) strictly on emeralds, and this one blows those other emerald books away.

 

[Indylady]

To Zeolite and Richard,
Do you have graphical representations, or even links to articles, of what you are discussing? Book references are appreciated, but since most Pricescoper''s don''t have these books, references only give us a snapshot. Both the link to the Bancroft/Hughes article and to the Wikipedia definition of gamut were greatly helpful.