A system of color description for gemstones
The importance of an understanding, an appreciation, and a keen perception of color to a gemologist should be obvious. From this point forward in the study of colored gemstones, it is necessary to describe colors encountered in the various trade grades and varieties of the major species. Terms such as "apple green", "leaf green", "cerise", "mauve" and "scarlet" mean one thing to one person and something else to another. Thus, some standardized color-nomenclature system is essential to learn the color differences that determine desirability and relative value.
Experience has proved that there are important dividends to the jeweler who makes a study of gemstone colors. In the western world, women tend to be much more conscious of color than men, with the result that they usually develop a much keener color perception. A study of color makes the student more aware of the colors around him, and a heightened perception of color is the usual result. This growing awareness of color nuances develops an ability to recommend color combinations to customers and to instill in them a deeper appreciation of the beauty of colored stones, this, in turn, means added sales effectiveness.
The study of color in all of its many aspects is not within the scope of this course, but the essentials to a basic understanding of a system of color terminology are outlined on the next several pages.
What is Color?
Briefly color is a sensation resulting from a stimulation of the
eye by certain wavelengths of visible light. To better understand this
phenomenon, the eye can be visualized as consisting of three types of
receptors, each sensitive to approximately one-third of the visible
spectrum, and a fourth type that registers brightness. If the three
color receptors are stimulated equally, a balanced sensation is created
that the brain interprets as white or colorless.
If they are stimulated unequally, the perception of color results. For example, when the eye is exposed to a beam of light consisting predominantly of the longer (red) wavelengths of the visible spectrum, the sensation is red; if the radiation contains equal intensities of both red and yellow, a color that is intermediate between the two (orange) is perceived.
An object by itself has no color; instead, its color, as perceived by the eye, is the result of the particular wavelengths of light that the object is capable of reflecting or transmitting. Those wavelengths not transmitted by the object are absorbed by it. This partial absorption of certain wavelengths of light by an object that produces its color is referred to as SELECTIVE ABSORPTION, and it accounts for the color of all objects other than luminous bodies.
To observer the typical color of a gemstone, it is necessary to illuminate it with a light source that contains approximately equal amounts of all the wavelengths of visible light. Daylight or its equivalent is used as the standard for color observation. Special artificial light sources that emit only a limited range of wavelengths (e.g., the sodium-vapor lamps used in some cities for street illumination) produce only a portion of the wavelengths found in sunlight; therefore, objects that normally appear as red or green in daylight for example, do not appear in their true color under these special lamps.
One of the keys to an understanding of the color of different
lies in the fact that most substances transmit and reflect almost all
of the visible wavelengths of light but some more readily than others.
The predominant wavelengths stimulate the corresponding receptors in
the eye to a greater extent, and thus we perceive the object as being
of that color.
Color and Hue
From the above discussion it can be seen that all sensation of vision is one bf color, and it is by this means that an object is distinguished from its surroundings. In this broad sense, white, gray and black are colors, since they contrast with their surroundings. Also, dark red (maroon), light red (pink), orange and reddish orange are different colors, because each is a different sensation.
Hue in its narrowest sense refers to all color sensations other than white, gray or black. In order to eliminate any ambiguity in the use of the world color, we will refer to all color sensations other than white, gray or black as HUES or as variations of hues. Thus a gem can be described as being white, black or gray or of various hues (blue, yellow, blue-green, etc.). Hues are sometimes called CHROMATIC colors to distinguish them from the so called ACHROMATIC colors of white, gray and black.
The term COLORLESS is applied to transparent (or, for all practical purposes, nearly transparent) substances that transmit equally well a balanced spectrum of visible light; for example, a diamond without body color and a pane of glass.
In the sun's spectrum, which can be observed by passing a beam of sunlight through a glass prism to separate the various wavelengths, the average person is capable of distinguishing approximately 128 different hues, ranging from violet through red. Various combinations of red and violet that are not found in the sun's spectrum but that are observed elsewhere in nature and in man-made products account for approximately 22 additional hues. In other words, the average person is capable of observing and distinguishing about 150 pure hues.
These 150 pure hues are not to be confused with the total number of colors we can observe, since any given pure hue can vary from light to dark and from dull to intense. The total number of variations, or COLORS, that are detectable by the average person is estimated at approximately 1,000,000. Fortunately, for practical purposes, we do not have to be concerned with such fine distinctions as would be involved in separating one million colors. Instead, we are only concerned with general classifications, each of which encompasses a large number of these colors.
To simplify the nomenclature, a given chromatic color is referred to by the pure hue it represents, in addition to descriptive prefixes that designate any deviation from the pure hue. The total description involves what are referred to as the three attributes of a color: HUE, TONE and INTENSITY.
As explained previously, hue is that attribute of a color that causes the eye to perceive it as differing from white, black or gray. It is this attribute that makes it possible to arrange colors in orderly sequence around the circumference of a COLOR WHEEL or COLOR CHART (see accompanying diagram). The simplest system of naming the intermediate hues is to use a method that gives a visual impression of them, avoiding the use of such indefinite terms as "apple green", "Venetian red" and "sky blue". For example, the hues from yellow to green on the color wheel are described as yellow, greenish yellow, yellow-green, yellowish green and green. By this method, 24 hues can be described and their relation to one another visualized easily. These distinctions are particularly important for grading emeralds, rubies and other colored stones.
Tone is that attribute of a color that causes the eye to perceive it as holding a position on a light-to-dark scale. To produce colors of different tones in paint, for example, different amounts of white or black pigment are added and it becomes lighter or darker. It should be noted that since white and black can be added to gray, it can also vary in tone. This is not true of white or black, however, since any variation from either one results in a tone of gray (one could not visualize black as being a "light" or "dark" black). Tone is particularly important in diamond grading, since the principal trade grades are based on total variations of yellow.
Having represented intervals of hues around a wheel, it is perhaps best to indicate intervals of tone by their position on the "spokes" of the wheel, as shown on the accompanying diagram. For the purpose of describing the total variations of gemstones, the terms VERY DARK, DARK, MEDIUM, LIGHT and VERY LIGHT are sufficient. This is an arbitrary division of the scale, of course, since there is a gradual darkening of any hue from white to black. A vast number of tones are possible, each producing a different color sensation.
Intensity is that attribute by which a color is distinguished as being vivid or dull and by which the eye perceives it as holding a position on a scale of vividness to dullness or grayness. The intensity of any hue can be lowered from a maximum by the admixture of gray. The intensity of gem's color is described by the use of the terms HIGH (or VIVID), MEDIUM or LOW, and additionally by the adjective GRAYISH or BROWNISH for still lower Intensities (the term brown applies to low intensities of red-orange and yellow).
Reference to the accompanying color wheel will show that each hue has another hue exactly opposite it. Pairs of opposite colors are referred to as COMPLIMENTARY COLORS. On the wheel, red, yellow and blue are the PRIMARY COLORS and their complements are referred to as SECONDARY COLORS. Pigments of any of these opposite colors that are used commercially in paints and in art work well, when mixed in equal quantities, produce gray.
It is interesting to note that if any intense hue is viewed for a short time in neutral surroundings and the eye is then shifted to a pure white surface, the surface will appear as the complementary color of the hue. This is easily explained by the fact that the sensation produced in the eye by visible light involves a chemical reaction: the subsequent temporary depletion of chemicals in the activated receptors of the eye results in these receptors being incapable of full response when exposed to the white background. Thus the normal simulation of the remaining receptors becomes, in effect, the equivalent of an exposure, to the opposite or complement, of the color just observed. Such an effect is called AFTERIMAGE.
Pigments and Absorption
The color of an object when illuminated by white light is, as stated previously, the result of the object's ability to selectively absorb, or subtract, certain of the wavelengths of the light reflected from, or transmitted through it. The resulting colors are called SUBTRACTIVE COLORS. The color circle is therefore a SUBTRACTIVE COLOR WHEEL. It does not convey the reactions and resultant colors that would be obtained if colored lights were directed at, and thus superimposed on, a white background that, in turn, would reflect all wavelengths directed at it. For example, mixing pigments of the three primary hues on the subtractive color wheel will result in black, since between the three all of the wavelengths of light directed at the mixture will be absorbed. Three light sources, however, each producing wavelengths that are representative of the three primary hues, if superimposed, will produce white, since, by addition, all of the wavelengths necessary to stimulate the three types of receptors in the eye are added together. A color wheel used to illustrate the effects of adding light from colored illuminants is referred to as an ADDITIVE COLOR WHEEL. It is not illustrated here, because it has no application in the classification and description of colored gemstones.
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