The Four C''s of Connoisseurship

[Sagebrush]

Here is a good question for you early risers,

In 2001 I introduced the idea of the "four c's of connoissurship", color/cut/clarity/crystal. Grading gems, particularly finer quality gems using only color/clarity/cut seemed to me inadaquate. Something was missing. The introduction of transparency or "crystal". The clue to this I found in the use by people like Tavernier of the old term now archaic term "water" as in "gem of the finest water" which is defined as color/transparency.

Most of my book's reviewers either passed over or seemed not to understand my point. How do you feel? Can a fine colored gem be adequately described without using the word "transparent" or some synonym? Is "crystal" somehow redundant?

 

[MJO]

Richard,

I definately believe you are right on crystal. I have seen many stones that there is a big difference in crystal. Mandarin Garnet is the one that comes to mind right away. I have stones different stones that are all VVS1 however some are more like fine Waterford Crystal and the others look like glass. All are native cut and close in color but to me that is the difference between fine and extra fine material.

 

[Sagebrush]

For those of you who have not read my book below is the content of a lecture I delivered at The American Gem Trade Association conclave in Tucson, February 2001:

Tucson Lecture
Connoisseurship; The Finer Points,
Crystal: The 4th C.
By Richard W. Wise, G.G.
2001


Crystal the Fourth C
Most approaches to colored gemstone grading rely on the four C’s, namely color, cut, clarity and carat. Color is normally divided into the categories of hue, saturation and tone. Cut addresses primarily the percentage of brilliance in the stone viewed face up and secondarily the proportions of a given gem, i.e. length, width and depth. And although some systems, including the one used by The Guide, distinguish between eye and loupe clean stones, clarity generally, at least in colored stones, addresses eye flawlessness, or the relative lack thereof, in a gem viewed face up. Carat weight, being merely a quantitative measure of weight, has no place in the discussion at all.

In Philosophy a distinction is made between conditions that are necessary and those that are sufficient. In the formation of a human embryo, the male sperm and the female egg are both necessary conditions. But, it takes two to tango. Both are necessary, but neither is sufficient. Only when they come together do they create the sufficient condition. The attempt to use the three C’s as the sole criteria for quality grading gems, there is a similar situation. All three—color, clarity and cut-- are necessary conditions, but taken together, particularly when grading higher quality gems they are not sufficient.

A Tale of Two Sapphires:

What is it that distinguishes a fine natural blue Sri Lankan sapphire from its heated counterpart? Answer: transparency, crystal or what gemologists call diaphanity. Few would argue the fact that the natural stone has a limpid crystalline quality that, to the expert eye, easily distinguishes the two stones. A heated stone may have exactly the same hue, saturation and tone, be perfectly clean, and be exceptionally well cut, but because heating tends to muddy the crystal it will often be less transparent. In fact it is not possible to verbally distinguish the two stones without using adjectives like transparency, limpid and crystalline, all of which are synonyms for diaphanity or what most dealers call crystal.

A Gem of the Finest Water

The crystal criterion is neither a unique idea nor a recent discovery. As early as the Fourth century BC, Kautilya the ancient Indian Machiavelli, lists among the qualities of a “good gem… (That it be) “transparent and reflecting light from inside”1. Sometime around 1433 The Chinese Admiral Ying-yai Sheng-lan uses the term water to describe a particularly “clean, clear” quality of Amber . The famous Seventeenth century gem merchant; Jean Baptist Tavernier, used the phrase “gem of the finest Water” to describe the finest diamonds and pearls he saw on his six voyages to India. In the sixth edition of the Dictionary of Gems & Gemology published by GIA, Shipley defines water as a term occasionally used “…as a comparative quality designation for color and transparency of diamonds, rubies and other stones…” Color and transparency together equal “water”. Shipley goes on to mention a hierarchy, first water, second water, etc.,so clearly the quality of diaphanity was recognized as an important and indispensable criterion in quality grading from early times.

In the grading of Jadeite reference is made to the three C’s (color, clarity, cut) and the two T’s (texture, translucency). To be classed among the finest, Jade must exhibit a “first class transparent texture.”

Although water is a rather poetic term, not easily quantifiable, I sometimes think that with our modern mania to reduce all terms to scientific precision we have lost much in the way of romance. Another more modern synonym for diaphanity is used to describe diamonds. The term is super-d. This designation, one that sounds about as romantic as a brand of motor oil, refers to antique diamonds from India’s legendary Golconda mines – stones that are reputed to be extraordinarily transparent.

Do ultra-transparent Indian diamonds really exist, many well known experts believe so. Benjamin Zucker suggests “Place a Golconda diamond along side a modern, recently cut D-colour diamond and the purity of the Golconda stone will become evident.”3” Mary Murphy Hammid in a essay on Golconda diamonds written for Christie’s maintains: “I’ve seen the incredible transparency that people say is characteristic of Golconda diamonds in stones the GIA graded G and H4,”. And why not? crystal and color are two distinct qualities. Super-d is an obvious misnomer. These diamonds are not ultra-white, they are ultra-transparent.

One might wish to argue that diaphanity is nothing more than a subset of clarity. In other words, that it is really extremely tiny inclusions that cause a stone to exhibit poor crystal. Perhaps the most famous example are the famous sapphires of Kashmir the finest of which exhibit a sleepy or fuzzy appearance resulting from light refracting through and reflecting from myriad tiny sub-microscopic inclusions. Some Sri Lankan sapphire as well as Tanzanian pink spinel will exhibit similar phenomena. In these examples it is clear that inclusions are the culprit, but diaphanity is really a distinct quality. When we say a stone is limpid, clear or crystalline, we are talking about a quality quite distinct and different from clarity.

From a strict grading perspective, the inclusions that create the fuzzy appearance in some gemstones are not visible or at least not resolvable under 10x and have little effect on the clarity grade given a particular stone. In the case of Golconda or super-d diamonds, the presence or absence of ultra-transparency would have absolutely no impact on the clarity grade listed on a GIA laboratory grading report.

Tiny inclusions are one, but only one, of the possible causes of poor crystal. Poor diaphanity has several causes. Consider a strongly blue fluorescent diamond. Such stones are often described as visibly oily due to a loss of transparency when the diamond fluoresces in daylight. Fluorescence will be noted in a grading report, but its presence or absence cannot be said to affect the stone’s clarity grade.

Many varieties of gemstones tend to lose something when viewed in certain lighting. Incandescent light is the usual culprit. Most varieties of tourmaline and garnet and some varieties of corundum seem to “close up” “muddy” or “bleed color” when exposed to the light of an ordinary light bulb. The exact cause of these effects is not well understood. The point is, the gem loses transparency and some of its beauty. Therefore another criterion is needed to properly quality grade gemstones. That fourth “C” is crystal.

Crystal must be judged in various lighting environments. Different types of light have distinct color temperatures that are measured by units, Kelvin. North daylight at noon, the traditional gemstone grading standard, is balanced between yellow and blue at 5,500 degrees Kelvin. As Kelvin temperature decreases, light becomes yellowier, and as the temperature increases the light becomes bluer. Incandescent or lightbulb light at 2,800 kelvin is distinctly yellowish. The lighting temperature determines the color of the light, and that in turn impacts the visual appearance of the gem being viewed in that light.

The tendency of gems to change appearance between natural daylight and incandescent light has traditionally been called bleeding. In blue sapphire, for example, one of the qualities that makes a Kashmir stone so desirable is that it doesn’t bleed color. Due to an absence of chromium, the color of a fine Kashmir sapphire will remain unaltered as the lighting environment is changed.

The visual appearance of other varieties of gemstones, including some ruby and sapphire and most varieties of garnet and tourmaline, will also change as the lighting environment changes. However it is not quite accurate to use the term bleeding to describe the result. I doubt if any jewelry professional who regularly works with colored gemstones has failed to notice these alterations. And although language lacks precision there is little choice but to use it to describe the visual effect, Tsavorite garnet seems to close up in incandescent light while rhodolite turns muddy and brownish. Green and blue tourmaline pick up a gray mask and appear dull and sooty like the chimney of an oil lamp. Pink to red tourmaline acquires a muddy brownish mask. Not all the effects are negative, aside from its loss of transparency, Thai ruby turns a purer red losing its purplish secondary hue when viewed in incandescent light.

The changes described affect not only crystal, but color (hue, saturation and tone) as well. Such effects are general, but not universal. For example, ninety-eight percent of all rhodolite garnet will muddy, turn brownish, losing both transparency and color saturation in incandescent light. This leaves only about two percent that retain both there color and crystal under the lightbulb. All other C’s being equal, if the stone is of high color, clean and well made, this two percent constitutes the crème de la crème of rhodolite garnet. The same may be said for pink tourmalines that do not muddy and tsavorite that retains its open color in incandescent light.

Historically crystal has also played a part in the discrimination of the finest pearls. Prior to the introduction of cultured pearls which are seeded with an opaque sphere ground from the shell of a freshwater mollusk, transparency or at least translucency was very much a characteristic valued in the finest pearls. In his Travels to India, the fifteenth century gem dealer J. B. Tavernier describes the world’s paramount pearl (circa 1650), a gem at that time in possession of a minor prince of Muscat. “This prince possesses the most beautiful pearl in the world, not by reason of its size for it only weighs 12 1/16 carats nor on account of its perfect roundness; but because it is so clear that you can almost see the light through it”6. Tavernier also repeatedly uses the term gem of the finest water to describe the quality of pearls.

As we have shown, diaphanity, transparency or crystal is a necessary grading criteria that deserves more than just a footnote in the discussion of quality in gemstones. Several factors including; sub-microscopic inclusions, ultraviolet fluorescence and the color of the lighting environment may impact on crystal. Thus crystal is a distinct criterion and it cannot be reduced to a subset of clarity.


The distinctions made in the course of this discussion are real in that they reflect observable phenomena that affect the beauty and desirability of gemstones. They are real also because they reflect demonstrable price differentials in the marketplace. From a grading perspective, crystal is a distinct and vitally important criterion without which it is impossible to adequately describe the finest gemstones. In short, crystal is the true fourth C of gemstone quality evaluation and connoisseurship.


In the first part of this lecture I discussed the critical importance of diaphanity, transparency or crystal in the evaluation of gemstones. In that section I suggested that “crystal” was the defining characteristic of the very finest in gemstones and, as such, is the true "fourth C" of gemstone quality evaluation. In this section the discussion turns to a close-up analysis of the three aspects of color--hue, saturation and tone, the relationships between these criteria, and the finer points of color discrimination in gemstones.

When we evaluate a colored gemstone, color is normally the most important of the four C’s. In the good old days it was said that in evaluation color was worth fifty percent of the equation, with clarity and cut each worth twenty-five percent each. Color itself, as we know, is sub-divided into hue, saturation and tone. Considering this triumvirate, hue is usually the most important factor and a necessary condition with saturation and tone running about neck and neck just behind. A ruby for example must be red, that is the necessary condition and the evaluation of that color was paramount. If it is not red it may be a beautiful sapphire but it is not a ruby.

Further complicating the analysis is the fact that viewed face-up, color in a faceted gemstone is far from uniform. It is a complex mosaic, a chromatic jigsaw puzzle a shifting crazy quilt of color, Some facets are bright, others dull; some one color, some another. Some portions of the stone reflect light, others appear dark. In order to fully understand, evaluate and communicate what we see in the face-up mosaic of color, three additional grading criteria are necessary--these are: dichroic effect, color bleeding, and the division of hue into preferred primary and secondary hues.

As every gemologist should know, light behavior, i.e. refraction and reflection are a function of proportion and cut. One light ray may enter, bend and exit through the pavilion of the stone, while another will reflect off the pavilion facets and exit through the crown. Other rays will totally reflect like a demented pinball all about the inside the gem, eventually exiting who knows where. The behavior of each light ray, the length of its path, and its eventual exit point determines what we call the face-up mosaic of the gem.

The length of the path that light follows determines the quantity of color we see. Saturation is the term used to discuss the quantity of color that we see. The longer the path light travels, the more color the ray picks up in its rapid passage through the gemstone. In addition, the ray also loses some color through selective absorption.. This explains why, even in a singly refractive gem, some facets may exhibit differences in darkness or lightness what we call tone or appear duller or brighter in saturation than other portions of the stone. In a doubly refractive gem, the scene is made more complex by the splitting of each ray into two components, each containing a portion of the visible spectrum. In dichroic gems, additional hues may be added to tonal variations, further complicating our faceup mosaic.

Dichroic effect is the general term that describes this phenomenon is not necessarily negative. Stones which show dichroic effect simply are those that show two or more distinct hues, or tonal variations of the same hue, when viewed face up.

Dichroic effect is a negative when the shift in lighting environment causes the gem to pick up a saturation modifier or mask such as gray or brown. A mask reduces transparency or crystal and dulls the color saturation of the gem. Some experts would argue that, in gemstones a single visually pure color are always the most desirable. That is to say, red stones should be pure red and orange stones should be pure orange and therefore any visual dichroism has something of a negative effect. Such generalizations are dangerous. In my view, dichroic effect has a positive effect on quality when the stone picks up a secondary hue that enhances and reinforces the primary hue. The classic example is when a green stone either emerald or tsavorite picks up a slight blueish secondary hue in incandescent light. Dichroic effect has a negative effect on the beauty of a gem when the stone picks up a secondary hue that dilutes the primary hue, as when a green stone becomes noticeably yellowish in an altered lighting environment. In some gem varieties such as andalucite, and topaz to name just two, the presence of visual dichroism is integral to the attractiveness of the faceted gem. In all cases, beauty is the ultimate criterion and each stone must be judged on its own merits.

Bleeding is an older term that can be defined as a specialized negative subset of dichroic effect. A gem is said to bleed color when it loses saturation/tone when the lighting environment is changed from natural daylight to incandescent lighting and vice versa. For example, a vivid blue sapphire of 80% tone bleeds to a blue of 50% tone thus loosing its richness when viewed in daylight. When we discuss bleeding we are talking mostly about tonal variations of the same hue. Bleeding is a common fault in blue sapphire.


Natural things rarely come in visually pure hues. For this reason connoisseurs divide hue into primary, secondary and sometimes tertiary components. For example, a ruby is never pure red; it is either violetish, pinkish or orangish red. The ish refers to the secondary or modifying hue. Secondary hues are also pure spectral hues or modified spectral hues such as purple and pink.

This approach to grading borrows liberally from The American Gemological Laboratory’s Colorscan grading system. The Colorscan system works well because it treats the non-spectral hues gray and brown not as hues, but as saturation modifiers or masks; the presence of either acts to dull and muddy, that is, to reduce the saturation of those hues that are present.

Among the most controversial of the finer points of connoisseurship in gemstones is the question of preferred secondary hues. Is a pinkish ruby preferable to a violetish ruby? This is a question that is often asked but seldom answered. In the December issue of JCK, Gary Roskin identifies pink as the preferred secondary hue in ruby . This is a courageous call, and Roskin deserves credit for having the intestinal fortitude to render an opinion. Unfortunately, I believe his conclusions are the wrong ones! Let’s see if an analysis of this issue can clarify a few of the points made so far.

Running the Gamut:
Normally gemologists measure tone (lightness to darkness) using a scale from 0-100%. Zero tone is equal to a totally colorless transparent medium such as ordinary window glass. Color scientists normally reverse this scale. As the tone approaches one hundred percent, the hue, whatever it is, becomes darker and darker until eventually it becomes oversaturated, i.e. black. Think of it as adding black paint to a bucket of paint of any pure hue. The more black, the darker the color gets until it eventually just blacks out.

Although scientific analysis is considered objective and connoisseurship subjective, science can, at times, be useful to the connoisseur. In this situation color science becomes an ally in the attempt to get a handle on the finer points of connoisseurship. There are five pure spectral hues: red, orange, yellow, green, blue. Color science has discovered that these pure spectral hues reach their maximum saturation at certain definable tonal levels.

According to color scientists, each hue achieves its optimum saturation at a certain tone. Color scientists call these optimum levels saturation gamuts. For example the transparent hue blue reaches its optimum saturation at about eighty percent tone . For most sapphire aficionados, the optimum tonal value for a fine sapphire falls between seventy-five and eighty-five percent. Eighty percent is the median--hitting right on the blue gamut. It is important to remember that although we talk about the three aspects of color as if they were separate qualities, in reality the are not. They are only abstractions. Saturation and tone are really two sides of a single coin. One cannot exist without the other.

One thing is clear. If we examine the optimum saturation gamuts for the five spectral colors we can see that a very close relationship exists between a hue’s optimum saturation/tone and the preferred tonal level for gemstones personifying that hue. This leads to a simple, perhaps self-evident conclusion--the brighter-hued stone is the better-hued stone.

Returning to ruby, the hue we call pink is simply the name given to light-toned red. In ruby, a pure spectral hue is preferred. That is to say the ideal color in ruby is red--the brightest, richest, purist red possible. Red reaches its optimal saturation gamut at about 75%, a rather dark tone. When we talk about pinkish red we are talking about a secondary hue that is, by definition, a light-toned modified spectral hue, in short--a light-toned red. In ruby a preferred secondary hue is, or should be, one that reinforces, one that does not dilute the primary hue. In this case a light tone added to a dark tone dilutes the latter--reducing the color saturation below its optimum gamut.

This is not to say that a pinkish-red ruby isn’t beautiful. My wife has a piece I made containing a highly UV florescent pinkish red three-quarter carat Vietnamese stone that is visible across a crowded room. Lighter toned ruby, particularly florescent stones, sizzle like beef fat on a hot grill! Also, if one wishes to appeal to authority, the 15.97 carat Caplan ruby, the most expensive ruby sold to date, is a visibly pinkish gem . However, if the optimum saturation/tone gamut for red is 75-80% how can a secondary hue that if effect lightens that tone be preferred?

In ruby, pink is the least desirable of the possible secondary hues. Which is the most desirable? We are left with orange and blue. Notice I didn’t mention violet or purple. A ruby can be either, visually, but the appearance is caused by blue mixing with the primary red, which produces either a purplish or violetish hue, depending upon the strength of the blue. Remembering the spectral hues--purple is a hue-modified spectral hue falling halfway between red and blue; violet is a hue halfway between purple and blue. Thus violet is further from red on the color wheel than is purple. Whether the ruby seems purplish or violetish depends completely on the strength of the blue secondary. Given that the ideal is a pure red, and purple is closer to red, less is more--purple would naturally be the preferred secondary hue.

“Asking to see the pigeon’s blood is like asking to see the face of God”.
Anonymous Burmese trader

On a trip to Moguk reported in this publication (see my “Along the Burma Road” issue? Date?) I saw a few natural slightly purplish “pigeon’s blood” rubies. Because the optimum saturation/tone gamut for purple is, at 80% tone, just slightly darker than red, the purplish secondary hue visually reinforced and richened the red primary hue.

This leaves us to consider orange. Orange as a secondary hue is rarely found in Burma-type ruby. It is much more characteristic of ruby from Thailand which is formed in the iron-rich soils of Chantaburi and Trat provinces in the central part of the country. Thai rubies tend to be dark in tone with a dark gray mask that visibly dulls the red primary hue. Orange is a spectral color that achieves its optimum saturation at fairly light tonal levels, perhaps 30%. Thus a bit of orange will tend to lighten, and to some degree, brighten the overall tone.

At darker tonal levels orange becomes increasingly brownish. In fact there is really no such thing as dark orange--darker toned orange is simply brown. Brown ”muddies” the red primary hue, thus reducing its saturation. This is the reason why, with the discovery of a new source of Burma ruby at Mong Hsu, Thai ruby all but disappeared off the radar screen. Thai ruby is often of a purer red primary hue than the Burma type, but the addition of brown so reduces the saturation that the stone appears dull as dirt when compared to gems from Burma.

As I said earlier, considering the sub-divisions of color, namely hue saturation and tone, hue is normally the defining criteria in judging the quality of the stone. Saturation and tone run a very close second. But, there are a few cases where hue drops to second place and either saturation or tone become primary. In other words there are a few cases in which the important thing is not what color it is but rather how bright and rich that color is.

Consider Paraiba Tourmaline, arguably the saturation success story of the 20th century. Discovered in 1989, and mined out by 1990, here is a gem that has turned tourmaline grading on its head. Tourmaline used to be the look-a-like stone. A gem that was judged on how closely it resembled other stones. The finest green was that which was closest to emerald, the finest red that which most resembled ruby. Paraiba tourmaline doesn’t resemble any other stone yet it has escalated in price from several hundred to tens of thousands of dollars in a single decade. The key to understanding this dizzying increase in price is to be found in Paraiba’s verbal description---“neon” or “electric blue”. Few gemstones achieve the intense saturation of Paraiba tourmaline. Perhaps the only contender, Burma Ruby, not surprisingly sells for a similar price.

The same is true for opal, as an old dealer once said to me; “opal is like a light bulb, the brighter it is the better it is”. Put less poetically it is saturation not hue or tone that is the central truth of connoisseurship in opal.

Tone is central in the evaluation of Aquamarine. It is fair to say that the darker the hue the finer the Aqua. This is despite the unfortunate fact that darker toned aquamarine is often noticeably grayish.

The foregoing has been what the late great media philosopher Marshall McLuhan called a series of probes. A probe is an attempt to get at the truth, a nod, a step in the right direction, if nothing else a stimulus to further thought. Much of what I have said is my own opinion and you are free to disagree. The criteria if have enumerated are real, visible and definable qualities found in gemstones. The understanding of and application of these criteria are among the finer points of connoisseurship in gemstones.

 

[Michael_E]

Richard,
Very interesting ! But you have forgotten the fifth "C".
CASH and lots of it! [$$)]

 

[mogok]

Hello Richard,

Nice post! Good way to wake up...
Well about the "crystal" I agree with your and think that I understand what you mean about it. In the Burmese gemological school I studied, there was: color, clarity, cut, carat and "luster" which in fact cover globally the same aspect as your luster. "Luster" is a way to evalute the transparency of the gem and the way the light is travelling inside the gem and is reflected by it... Some other people I met and speak with were speaking about "life" that also include some cutting concepts as life is a global concept to evaluate the artistic quality of the game between the stone and the light. window and extinction kill the life...
I like your "crystal" concept, I like the Tavernier "water" and the "life" also... The term "luster" is less clear.

But may be not enough people were given the possibility to see a high crystalline quality gem and to compare is with others... But I think after few minutes of thinking reading your book and some experimentations with actual gems, your concept is clear and useful.
But as for many things, people need to be open minded, and to do their homework as you say very often.

All the best,

 

[spinel]

Thank Richard, you are absolutely right with the crystal criterion in gems grading.
One will see that more clearly with a wonderful feeling when holding such a stone in hand!
The crystal quality is more easily found in spinels but is extremely rare in rubies.

This is a water pink spinel cut from a see-thru rough that Vincent has seen but maybe did not remember

(in Dong's house with JB)

 

[valeria101]

I found "water", "crystal", "life" or "light" more suggestive than the CCCC... It doesn't hurt that those old terms got into my vocabulary before the four Cs which I then found too dry and analytic. The usefulness of the four Cs to formalize metrics and quantify grades is glaringly obvious, but the distance between the image they convey and visual perception leaves allot to the imagination.

Compared to the old quality concepts, it is easier to talk about 4Cs in an abstract way (without the concerned object at hand, as it happens on this forum) and harder to use them to convey the visual impression of the real thing. The old concepts may be difficult to quantify, but easy to identify as attributes of a real object held in hand. I am not sure if the four Cs have improved the ability to communicate the impression and desirability of gemstones, simply because much of their desirability remains closer to metaphor and aesthetic sensibility - just like "water" and "light" are metaphors not strictly defined physical qualities themselves.
There seems to be one such metaphor surviving all too well aside the four Cs - it is "fire" used about diamonds. That one is getting a quantitative translation of sorts. Quite often I wish that "water" and "life" got such translations so that the modern technical communication does not kill the "natural language" formed through centuries of trade.

It is ironic, that the current refinements of diamond cut quality research use models of sensory perception to associate quality to purely quantitative technical metrics. This is very much in the spirit of those old attributes that made direct reference to perception. Along this line, there may actually be some need to look back - as "Secrets of the Gem Trade" does - and synthetise those enduring concepts of quality and value out of the overly analytic four Cs. And I am not just saying this just from the perspective of a traditionalists' as wishful thinking.
The web has a great way to produce snapshots of communication styles about everything, including gemstones. The composite quality scale constructed at Cherrypicked.com and followed by others appears as a response to the need of integrating the abstract four Cs back into a mental image. This is not unlike the function of metaphors (as form of communication) - and that's exactly what the old words used to describe gems were. After all, perhaps not all can see perfect gems, but fire and water and light are definitely global common knowledge.

 

[Translucent]

Richard,

Enjoyed your generous post alot!

After playing with a lovely 10 carat Burmese sapphire last year, I now know why I kept looking at it every 5 minutes
while it sat on my desk. It displayed that "water" quality! So now I have a word for it. I kept trying to
describe it to friends but just couldn''t do it.

 

[valeria101]

These two are both "SI2" clarity... case in point, I think.

(the pictures are not mine)

 

[Translucent]

Ana,

Is it possible that the second stone is highly fluorescent? And that is what is giving it its milky sheen?

 

[valeria101]

Date: 5/10/2005 2:25:23 PM
Author: Translucent

Is it possible that the second stone is highly fluorescent?

I have no seen the lab reports and only ever knew the color and clarity grades. However, that stone does not have any other inclusion to justify the SI2 grade but the coulds - no wonder there''s so much of them. If there is fluorescence on top it can only add up. If I ever retrace the source of these pictures (some seller, just didn''t note which), it would be straightforward to ask. Those photos were saved some time ago for the glaring example of low clarity based on clouds.

 

[partgypsy]

Thank you for the article, I found it very interesting. I have heard that very fine emeralds are called "gota de aceita", or oil drop, perhaps visually a similar phenomenon to very fine burma rubies or sapphires.
I have not experienced enough gems to have needed to use the term "water" to distinguish fine from extremely fine gems, but have found some reference to the crystalline structure is informative for explaining, well what is a gem? I was trying to explain to my husband the difference between minerals and gemstones, that all gemstones are minerals (well, except for diamonds, and perhaps organic gemstones) but were a subset of minerals, specimens with most pure crystalline form, some which are colored by various impurities. This was not to explain the level of fine gemstones from cream of the crop, but why someone would pay $500 for as my husband would say "a rock?".

 

[Richard M.]

Date: 5/10/2005 2:25:23 PM
Author: Translucent


Ana,


Is it possible that the second stone is highly fluorescent? And that is what is giving it its milky sheen?

Either that or it''s been through a fire. Fire-frosted diamonds can usually be restored to transparency by a simple repolish.

Richard M.

 

[Translucent]

Richard M.,

A fire? Really. How interesting and surprising.

I sure am learning loads of stuff from this forum. The problem is that I have access to wonderful stones, but have no
solid foundation of info. about them. I''m sort of working my way down from the top. Very odd experience for me.

 

[Richard M.]

Date: 5/10/2005 4

9:38 PM
Author: Translucent


Richard M.,


A fire? Really. How interesting and surprising.

Being pure carbon, diamonds begin to burn and convert to carbon monoxide and dioxide at between 700 and 900 degrees C. (1290-1650 F.) depending on the quality of the stone. In the 17th century a "magician" made a diamond to vanish at a famous public demonstration in Italy by heating it with a burning glass, or so it''s said. English physicist Robert Boyle discovered just a few years earlier that diamond disappears into smelly vapors when heated to high temperature.

 

[MJO]

Date: 5/10/2005 4:46

8 PM
Author: Richard M.

Date: 5/10/2005 4

9:38 PM
Author: Translucent


Richard M.,


A fire? Really. How interesting and surprising.

Being pure carbon, diamonds begin to burn and convert to carbon monoxide and dioxide at between 700 and 900 degrees C. (1290-1650 F.) depending on the quality of the stone. In the 17th century a ''magician'' made a diamond to vanish at a famous public demonstration in Italy by heating it with a burning glass, or so it''s said. English physicist Robert Boyle discovered just a few years earlier that diamond disappears into smelly vapors when heated to high temperature.

Expensive gas.

 

[Sagebrush]

Ana,

The stone on the right would, today, be considered a "white" diamond but I take your point. One stone is clear as, well, crystal and the other is slightly turbid. But, from the perspective of clarity they are identical. I, of course, mention this in the section on clarity vs. crystal but somehow the reviewers missed it. Genis said (Rappaport News) that "crystal" would be difficult to quantify, duh, why? The G&G reviewer suggested that "crystal" should be a sub-set of clarity.

To me, it all seems self-evident. That is, if one were to sit down and write a paragraph describing a top quality gem, say a ruby. It would be impossible to do so without using the word transparent, diaphanous or another synomym.

 

[Richard M.]

Richard W.,

I’ve studied your piece on "crystal" and it offers much to think about. I don''t question that the term is in use among gem dealers -- I''ve run into it myself -- nor that it might have a specific and useful meaning under certain conditions. But you yourself question whether it’s redundant.

Will you explain to me how it differs from the term “transparency” as used in gemology? When I did my GIA colored stone work I was taught transparency''s the term that should be used to describe “the quality and quantity of light that is transmitted or allowed to pass through [a gem].”

Basically, my instructors said the quality of light transmission is affected by “minute imperfections or peculiar structure that {light} encounters during its passage through the stone.” It seems to me the word, perhaps used with modifiers, should quantify the distinction between the transparency of heated and non-heated corundum, i.e. your “crystal.”

The quantity of light transmission (i.e. transparency) is affected by absorption of light waves, as you discuss. There are the familiar subdivisions of transparency: transparent (with the disclaimer that no gem material is totally transparent); semitransparent, transclucent, semitranslucent and opaque. Are you suggesting additions or changes to this hierarchy?

Vincent mentioned luster in connection with your comments. In my experience luster has to do with a gem’s surface reflectance based on its nature or the quality of its polish -- something I know a lot about being a gemcutter. It’s defined by GIA as “the appearance of a surface when it reflects light directly to the eye” and is subdivided into the familiar “metallic,” “adamantine,” “vitreous” and other categories. A bad polish might result in a stone with less than ideal luster for its type, thus also affecting the quality/quantity of transmitted light. A good loupe or even a critical eye should reveal that problem.

Richard M.

 

[Sagebrush]

If you read the article closely you will see that I am simply saying that transparency, diaphanity, water or crystal is an indispensable, a necessary criterion in the grading of gems. that the three C''s, color/clarity and cut are insufficient and a 4th C namely "crystal" is necessary to completly describe a fine gem.

 

[mogok]

Vincent mentioned luster in connection with your comments. In my experience luster has to do with a gem’s surface reflectance based on its nature or the quality of its polish -- something I know a lot about being a gemcutter. It’s defined by GIA as “the appearance of a surface when it reflects light directly to the eye” and is subdivided into the familiar “metallic,” “adamantine,” “vitreous” and other categories. A bad polish might result in a stone with less than ideal luster for its type, thus also affecting the quality/quantity of transmitted light. A good loupe or even a critical eye should reveal that problem.

Richard M.

Hi Richard M,

Yes in "English" luster is a term used for the gem surface reflectance based on its nature and the quality of the polish, but in "Burmesenglish" or in "burmglish" it is also linked with the transparency as there is the outside luster (the light reflected by the crown surface that does not penetrate the stone and then the inside luster for the quality of the light reflected by the pavillon. Now when you consider the inside luster aspect, you understand quite easily that if the gem is not very transparent, there will be a problem. The term "luster" used by the Burmese that were my teachers, and I think that this concept is used everyday by the MGE (Myanmar Gem Enterprises) gemologists is in fact not really the "western luster" but a more global concept that deal with the quality of the relation between the light and the stone. In fact when my Burmese teacher was speaking about a good "luster" for a stone it was meaning that the stone was highly transparent, well orientated crystallographically and usually with a good polish...
Spinels globally had most of the time a higher luster than ruby... It is not true if the term luster is only link to surface reflectance... But the fact is that spinel is single refractive and so does not have this haziness we can find in ruby or more in zrcon or peridot and also that many spinel are highly transparent...
It is the reason why I link this term to Richard Wise "crystal" or Tavernier "water".

Regarding to that I think that terms like crystal or water are better as they are somewhere less precise.
This is may be why some people does not understand well. There is this different way to define the position of a mosquito flying in a room between a typical western and a typical oriental approach. A cartesain "greek" will define the position using a formula that will try to pinpoint the mosquito... Finally as the little beast is flying quick as the moment they say its there it is not any more. Now an asian will show an area.
Westerners try to be very precise, Chinese are more artistic... if they use the same word, the word sometimes does not have the same meaning. Thats a linguistic problem as a writter try to express one idea and the reader can understand the things differently.

From what I had understood of Richard "crystal" it was a vision of the thing very close to the Burmglish "luster"... But not really the GIA "luster"...

All the best,

 

[Sagebrush]

Yo,
This "luster" issue is a good example of the sort of linguistic confushion that has plagued the gem-world for too long. This is partly the result of the fact that the trade has been shrouded in secrecy for so long. I too have heard Burmese dealers use the term luster as a synonym for "brilliance" which makes sense given Vincent''s explanation. The problem is further confounded, in Vincent''s case, by being funneled through the mind of a native-French-speaker.

It is only in the last decade that hue/saturation/tone have become standard terms and some dealers still confuse the latter two.

It is also "clear" that the Burmese term luster=transparency=crystal. In the evaluation of Burmese jade, transparency or crystal is part of the "three Cs and 2 t''s" color/clarity/cut; transparancy/texture. Also, in the evaluation of emerald you hear the term crystal or crystallization which refers to the gem''s transparency. My point at the beginning was simply that it is a necessary criterion in the evaluation of all non-opapque gems, particularly finer qualities and should be so stated.

 

[mogok]

Hi Richard,
Thats a good summary!
LOL...
I hope that my "bastard English" is not too bad and too confusing for you native english speaker guys. But the fact is that I''m not a native Englsih speaker, I''ve learn most of my english in Ireland and Wales, then learn gemology from Burm-glish speaking people and now with my team of Thai-glish speaking gemologists and world-glish speaking students, I''m trying to improve my english with mainly American and Aussies in some gemology forum...

Richard, you could add "crazy" before your "mind of a native-French-speaker"

Note: The "bastard English" term related to my French-thai-Burm-glish" is not from anybody present in this forum but from a good friend willing to help me to write and speak as good as he does.

Outside of this funny stuff, you are right terminology is a huge problem in the gemological field... if english is an international language the same word does not have the same meaning for everybody... already between English and American-English its a mess but went you try to explain in english something to a Thai, an american , an Indian, a chinese, a brazilain, a kenyan, and may be lets say an Iranian... How can you hope everybody to understand the same even if they all speak english. The problem is that we non native english speaker we most of the time translate in our language and then the meaning can be slightly changed!
There si a lot of work to do...

All the best,