(via Gemological Digest, Vol.2, No.3, 1988) William J Sersen writes:
Abstract
There is today no standardized nomenclature for color-related terms in use in the jewelry industry. It seems that every dealer and colored stone grading system designer utilizes his own version of nomenclature, often defining the same term in different ways. The net result is a lack of communication within the industry on matters pertaining to the quality grading of colored stones.
After describing some of the difficulties that can do arise from nomenclature use/misuse, the author proposes a set of terms to standardize gemstone and color descriptions. Only by adopting a standardized color nomenclature will effective color communication in the gem trade become a reality.
Introduction
Imagine for a moment that you are listening to someone describe their pet. Judging from the words they choose, you assume it is a cat they are talking about. You are later shocked to discover that their pet is in fact a dog.
Unfortunately, the current state of color communication among gemstone buyers and sellers is at times not far removed from this cat versus dog analogy. Just suppose that you see a colored stone (any colored stone will do) described on a price list as ‘fine color’. Upon actually viewing the piece, you discover that the seller’s concept of ‘fine’ corresponds roughly with your concept of ‘good’, ‘fair’, or perhaps even ‘poor’. In other words, what you thought was going to be a pedigreed cat has turned out to be a rabid dog.
As this article will demonstrate, a standardized and well-defined nomenclature of color-related terms is necessary before there can be effective color communication in the jewelry industry.
Color Terms Now Used In The Trade
Of the many colored stone grading systems that have appeared over the past ten years, some have used the same terms (tone, pleochroism, etc) to communicate different or slightly different concepts. In addition, many in the trade have coined still other terms to describe color attributes of colored stones. This creates confusion when people try to compare one system with another, or communicate with a dealer in the language of a grading system with which he is not familiar. Attempts at comparing different colored stone grading systems have never been easy; the lack of nomenclature standards has made this task even more difficult.
Before effective color communication is a reality, there must be a common language of color related terms in use both by people in the gem trade and by those involved in designing and using grading systems. That is, nomenclature must be defined in a consistent manner, without overlap, inconsistency, vagueness or redundancy in meaning. A standardized nomenclature of this sort would facilitate user comparisons of different grading systems. Users could then learn more than one system (if desired) without having to tackle yet another ‘dialect’ of color terms.
Typical examples of terms used inconsistently or redundantly are ‘overtone’, ‘pleochroism’ and ‘secondary color’. Let’s apply those terms to red spinel and ruby. One colored stone grading (AGMS:Valente, 1986) would have us think of the orangy red presence seen on certain facets in many red spinels as pleochoism, despite the fact that the pleochroism is possible only in DR stones, spinel being SR (color differences on different facets in spinel and other singly refractive gems are actually due to differences in path length or color zoning, not pleochroism). Other people might label that orangy red presence ‘absorption color (GIA: 1983) or ‘overlay color’ (Huffer: 1983), as opposed to the red ‘dominant’ color, itself sometimes called ‘primary color’ (Huffer: 1981) or ‘main color’ (Huffer: 1983; Ramsey: 1985). True color differences due to pleochroism (seen face-up on many rubies) are variously called ‘secondary color’ (Huffer: 1981), ‘pleochroism’ (AIGS and GIA) and ‘overtone’ (Ramsey: 1985), the latter suggesting the orange is superimposed over the entire crown. Overtone has also been used to designate any colors that are seen in addition to the stone’s main color (Ramsey: 1985).
Matlins and Bonanno (1987) refer to the ‘purplish cast’ (in actuality, a low saturation red) on some Thai rubies as an ‘undertone’. One American dealer recently suggested to the author that this was an incorrect use of undertone, the proper designation being ‘overcast’.
Besides referring to pleochroic color differences, secondary color has at times been associated specifically with ‘girdle facet color’ (color on facets near the girdle, viewed face up) and ‘window color’ (color on facets which show a window, viewed face up; AGMS: Valente, 1986). Varley (1980) say of secondary color that ‘the term is largely obsolete.
‘Body color’ is a prime example of a term used differently by different people. It has been thought of as a window color (Ramsey: 1985), ‘pavilion color’ (the color seen on the pavilion when a gem is placed table down; Yu: 1980) and ‘overall crown color’. Ramsey (1985) goes so far as to say that body color can be assessed by window and pavilion color, an interesting comment in that with colored stones these two colors are usually quite different.
‘Tone’, which GemDialogue has called ‘zone’ (Huffer: 1983), is another word used inconsistently. As we will see later, tone (in gems) usually describes the degree of lightness to darkness of the overall stone when viewed face up. But, this word has been used to denote ‘the amount of total color in a stone,’ a definition that misleadingly overlaps in meaning with ‘saturation’.
‘Saturation’, by the way, is called ‘intensity’ by some (AIGS, formerly) and ‘chroma’ by others, though there are those who relate the effect to ‘modifier’ (AGMS: Valente, 1986). The latter term describes degrees of desaturation, as evident by the amount of existing ‘gray’. GemDialogue has used a similar desaturation concept, known as ‘color mask’ (Huffer: 1983). Color mask, together with ‘brightness’, equates roughly in that system with the GIA’s concept of saturation (GIA: 1983). Even so, while some systems associate desaturation with gray or brown (Huffer: 1983; GIA: 1983), others associate it with gray only (AGMS: Valente, 1986), contending that brown is not a desaturant.
Are you just a little bit confused at this point? To simplify matters, I have listed but a few of the terms currently in use. The list of candidates for inconsistent/vague/redundant nomenclature usage goes on and on….
Why Do Nomenclature Problems Arise?
Dealers and gemologists together have described/created a plethora of technical terms—many of which have been borrowed or adapted from terminology in use by color scientists—for their respective methods of grading colored stones. As such terms are not always defined in the same way, defined unclearly, or constitute synonyms, it is no wonder that color communication has suffered; it is inevitable that it would suffer under such circumstances.
We can take some solace in fact that nomenclature problems are not limited to gemology and the gem trade. The development of meaningful semantic systems for describing sensory perceptions has long been a matter of concern in other fields as well, the audio and computer industries being notable examples. Let us take, for instance, the term ‘artificial intelligence’ as applied to the computer world. The question of whether it is misleading to use the word ‘intelligence’ in this context has been the object of much debate by computer people; at least one software engineer has suggested ‘artificial instinct’ as the more appropriate designation. If industries on the whole were always in agreement on the definitions and applications of technical terms, then there would be less need for organizations like ASTM (American Society For Testing And Materials), NBS (National Bureau Of Standards) or ANSI (American National Standards Institute).
Nomenclature problems in industry are further exacerbated by the tendency (often of marketing people) to believe that if the name of something is changed, it will somehow change or improve the products salability. Does the word ‘Bufferin’ sound better to you than just plain “Aspirin’? Is Tanzanite somehow more palatable than Blue Zoisite? Similarly, is ‘zone’ more appropriate than ‘tone’ for describing essentially the same thing?
Does The Answer Lie In Objective Grading?
Certain gemologists have stated that many of the present ills of colored stone grading will be removed once ‘objective (i.e.’machine’) grading is perfected. While such a goal is certainly desirable, can one really say that any color related term is objective? For that matter, does the often heard expression, ‘objective colored stone grading’, actually mean what is implies?
The Merriam Webster Dictionary (1974) defines objective as ‘…existing outside and independent of the mind….treating or dealing with facts without distortion by personal feelings or prejudices..’ Subjective is conversely defined by the same source as meaning ‘…..of, relating to, or arising within oneself or mind in contrast to what is outside…Personal…’
Now when someone makes a visual assessment of a gem’s color, is that not what a somewhat personal and hence, subjective analysis of the color? ‘Visual’ means the human eye/brain combination is a conditioning factor. Devices such as computers, image grabbers and high resolution color monitors may play the secondary role of electronically collecting, storing, sorting and ‘pigeon-holing’ color date; this does not, however, negate the eye’s role as a determinant or conditioning factor with the aid of which a grade is ultimately derived. While we certainly do look forward to the day when an electro-optical device can replace the human eye, such a day seems quite far off. That this is so is shown clearly by the fact that there is no color monitor or photographic apparatus available today which can approach the human eye/brain in its ability to resolve images. Has anyone ever seen a video image or photograph that was not immediately recognizable as such? If you have, then let us know. The fact is that the resolution ability of the eye is far greater than any device yet devised.
Can color scientists other professionals shed further light on the matter of the role of the eye? Yes they can. Many agree that people with so-called ‘normal color vision’ do not all perceive identically the full range of colors or more specifically, subtle color blends. Moreover, different ‘color normals’ vary in their aptitude to detect small color differences. The ASTM people (1987) go so far as to say that ‘persons employed in visually evaluating color differences should score near or within the ‘superior’ range of color aptitude (on Farnsworth-Munsell 100-Hue Test).
In other words, using the human eye to distinguish and assess subtle color differences does not appear to be an exercise in objectivity. We therefore submit that given the present state of colored stone grading, the phrase ‘objective colored stone grading’ should be avoided as it carries implications that are the least misleading. When instruments alone can accurately color grade gemstones with near 100% repeatability/reproducibility and without assistance from the human eye, then we can consider using the term ‘objective grading’. Unfortunately, this instrument does not yet exist. If and when it does appear, colored stone grading will become more a ‘science’ than ‘art’ and individual color terms can then assume a truly ‘objective’ status.
A Closer Look At The Main Terms
So, who is correct when assigning/defining color related terms for gem grading purposes? To help answer that question, let’s examine more closely the definitions of the major color attributes—the concepts of hue, tone and saturation—given by thirteen sources consisting of color scientists, gemological institutes, gemologists and trade journalists alike.
Hue
All sources use this term and agree on its broad meaning: ‘the position of a color on a color wheel’ (red, orange, yellow, green, blue, violet and purple, or colors intermediate between adjacent pairs of these). This ‘hue position’ includes pure spectral colors, color blends and so-called additional colors. Depending on the source, blends and additional colors are sometimes referred to as overtones, secondary colors or pleochroic colors. Some use the term pleochroism in the strict gemological sense (viz, GIA and AIGS); others (AGMS: Valente 1986) apply it more loosely. (The term ‘purity’ is often used in very different ways. Some (AIGS) use it to describe the location of a hue on a color wheel, with a pure red, for example, falling exactly midway between orange on the one side and purple on the other. Others use purity to denote the saturation of the hue; ‘pure’ red being slightly saturated red.)
Tone
Different sources use different terms to convey this concept. Six sources prefer using the word ‘tone’; four prefer ‘lightness’; three prefer ‘value’. Tone, lightness and value are at times used interchangeably.
Unfortunately, not all sources are consistent in the way they think of tone/value/lightness. The common and consistent denominator is relative darkness or lightness, with some (viz, Arem, Metheun and AIGS) describing the concept as a function of overall reflectance or transmittance (or conversely, overall absorption). Colorless (or white for opaque stones) would represent the lightness tone (zero), as none of the light striking the gem is absorbed. The greater the percentage of light absorbed, the darker the tone, with black having the darkest tone (100) as none of the light entering the gem is reflected back or transmitted through to the eye. As the term is used by AIGS, a zero tone (colorless or white) represents 100% absorption. All other colors would have tones between zero and 100.
In defining tone as the amount or quantity of total color in a stone, Huffer (1981, 1983) and Farrell & Thomas (1983) confuse the issue, as most people would associate that definition of tone with saturation than tone. Indeed, that definition of tone corresponds almost literally with Billmeyer & Saltzman’s (1981) definition of chroma (saturation).
Some associate tone/value/lightness intimately with saturation. Manson (1982) describes ‘tone’ a combination of saturation and blackness, though he elsewhere implies that tone is merely a synonym for blackness. AGMS (Valente, 1986) thinks of tone as the sum of value and ‘modifier’, the latter term meaning lack of saturation as evident by increasing presence of gray. Elsewhere, they note that tone is a combination of chroma and value. Varley (1980), in speaking of lightness compared with black or its darkest value…compared with white,’ is consistent with AGMS’(1986) definition of tone (=value + modifier).
Arem (1987) is one of the few who defines lightness/value without specific reference to chroma/saturation.
As if the issue were not complicated enough, the color scientists Overheim & Wagner (1982) observe that synonyms used for Munsell ‘value’ also include brightness, brilliance and luminosity. In the opinion of AIGS, there is a certain relationship between tone and saturation. In all cases it is correct to say that, if tone is defined in terms of overall reflectance or transmittance, increases in saturation always result in increases in tone, because higher saturation always result in increases in higher absorption. However, and it is here that confusion often exists, increases in tone do not always result in higher saturation. It is perfectly possible to have a very high tone (due to high absorption – low reflectance/transmittance), but still have a low saturation. Increasing saturation always result in increasing stone, but increasing tone does not always result in increasing saturation.
Saturation
Though the majority of sources prefer the term ‘saturation’ that term is often used interchangeably with intensity and chroma. The common denominator in most instances is the idea of color vividness. That is, how much color is present? How vivid is the color?
Some associate intensity/saturation/chroma with color purity, in the sense that the more pronounced the saturation, the more pure is the color. They think of ‘purity’ as freedom from color dilution. But, dilution by what? Here is where there is some disagreement.
Overheim & Wagner (1982) speak of dilution as freedom from black, white, gray. This is consistent with Varley (1980). However Brownlow (1983), in his article ‘Color Grading Terminology Unscrambled’, scrambles the issue somewhat when stating that a highly saturate color lacks dilution by white, black, gray and other colors. He gives as an example a yellowish green emerald, which is less saturate than a pure emerald green, saturation referring to the purity of a color. If saturation is just purity (in Brownlow’s sense of the word), then a pale yellow sapphire, with no color but yellow present, should be considered as having maximum saturation. The problem, of course, is that most people would not consider such a stone ‘vivid’ in color.
Many sources speak of saturation in a way that associates it with the absence or presence of gray, or in a few cases, gray or brown. Generally, the word gray simply refers to the amount of saturation. For example, the more gray a stone the less saturate or vivid is its color. AGMS (Valente, 1986) uses the word ‘modifier’ to describe relative degrees of color saturation in accordance with amount of gray present. Though not using the word ‘modifier’, the GIA (1983( similarly associates more ‘gray’ with the lower saturation levels.
For Metheun (1983), intensity or saturation means the density of hue present. ‘The denser the color appears, the more distinct is also the hue. A color with a distinct hue is described as being intense or saturated’. This is consistent with the AIGS definition of the term, ‘saturation’ referring to the vividness or intensity of the hue.
Analysis
With few exceptions, there is consensus on the definition and use of ‘hue’. But, there is inconsistency in how the concepts ‘tone’ and ‘saturation’ are defined. Specifically, the confusion seems to lie in defining the exact relationship of tone with saturation.
The GIA (1983b) says of tone:
‘Estimating the tone of low saturation stones can be frustrating at first. Because they tend to look grayer (or browner), it is natural to want to call them dark. Don’t confuse low saturation with dark tone. Instead, try to imagine low saturation stones as if you were looking at them on a black and white TV. Disregard the strength of hue and just ask yourself where the whole stone would fall on scale of light to dark.’
The statement: ‘don’t confuse low saturation with dark tone’ is simply not always true. Intensity/saturation/chroma—whatever you want to call it—refers to a scale of color vividness. Some ‘dirty green’ tourmalines, indicolites an many Australian blue sapphires have an overall dark appearance (= dark tone) and at the same time show color flashes of relatively low saturation. That is, the flashes are by no means strong or vivid.
Color plates or chips are useful for illustrating the concept of dark tone accompanied by low saturation. We start by selecting two plates, each containing the same amount of chromatic ink. The plates are chosen such that the first has little or no achromatic (gray) ink; the second plate is overprinted with so much gray ink as to give it a very dark (inky) appearance. The hue will appear less distinct or vivid on the second plate. The second plate is less saturate and of darker tone than the first.
It is true that three dimensional colored stones seldom display the same uniformity in color one finds with chips or plates. This is partly why one judges hue position and saturation from the flashes showing brilliancy and tone from the overall (face up) stone. With our green tourmaline, indicolite and sapphire examples, a very dark overall tone happens to correspond with relatively unsaturated brilliancy flashes. Type ‘D’ ruby exemplifies dark tone accompanying highly saturate brilliancy flashes.
Manson’s (1982) description of tone as the sum of saturation and blackness is a little confusing, in that many would think blackness and tone are the same anyway, as Manson himself seems to imply at one point in his article. So, do we infer from this that tone = saturation + tone? What exactly does that mean?
Conclusions
The research and development of colored stone grading systems is a fairly recent activity. Only the naïve would consider these systems—including that used at AIGS—as more than crude attempts at addressing the problems at hand: crude and very subjective. That so many color related terms have sprung up has a positive aspect. Marketing motives aside, new nomenclature does not arise without thought why that nomenclature is needed. This at least indicates that people are thinking of the subject. Regrettably, though, more people seem to think of the subject when colored stone prices are rising than during periods of price decline. Perhaps that is just a coincidence?
The jewelry trade must continue to research and develop colored stone grading techniques. Yet at the same time, efforts must be directed toward a standardized nomenclature so that these techniques/systems are more readily comparable and mutually translatable. Only when it becomes relatively easy to compare grading techniques will end users take the time to do so; and it is the end user who should have the final word on the matter. Standardized nomenclature will make the task all the more easy for that person.
A first step towards nomenclature standards is to get back to the basics. This means using terms that have a solid basis in color science, are familiar to the majority of gemologists, lend themselves minimally to misrepresentation and, ideally have prior acceptance by recognized color standards committees, such as ASTM.
Toward meeting these criteria, we suggest universal and consistent use of hue, lightness and saturation for describing the color appearance of gemstones, defined as follows:
Hue: The position of a color on a color wheel (red, orange, yellow, green, blue, violet, and purple, or colors intermediate between adjacent pairs of these).
Lightness: The relative darkness or lightness of the hue as a function of overall reflectance or transmittance (or conversely, overall absorption).
Saturation: The vividness or intensity of the hue.
All three terms are included in ASTM’s E-284 Standard Definitions of Terms Relating to Appearance of Materials. The majority of colored stone grading system designers and color scientists use hue, and we are generally consistent when defining it. Most of these same people prefer the term saturation to express the concept of color vividness. Lightness, it is true, is cited by only two of our gemological sources (Arem and Brownlow). But, the term is used extensively by color scientists, and does not lend itself to readily to the kind of misinterpretation one finds with tone, lightness being an almost self-explained term.
We propose that use be discouraged of terms like modifier, zone, overtone and body color. The first two are little more than synonyms for existing terms (saturation and lightness, respectively); the last two have always meant far too many things to too many people. None of the four have any widespread use in color science or are recognized by ASTM.
The term pleochroism or pleochroic color (s) seems preferable to other color descriptive designations like secondary color (which could refer to just about anything) to describe that color phenomenon. However, in keeping with standard gemological usage, pleochroism should be applied only to doubly refractive stones which do display visible pleochroism in the face-up position, as confirmed by examination through a calcite (not Polaroid) dichroscope.
It is the author’s firm belief that if progress is to be made in the field of colored stone grading, the gem trade must first take steps to standardize the terminology used in describing the color appearance of gemstones. Without a uniformly defined and applied color nomenclature, effective color communication will remain but a pipe dream.
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