Today, with "Divalenza", we are talking about colored gemstones. The importance of an understanding, an appreciation and a keen perception of color to a gemologist should be obvious. 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.
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.
Divalenza would explain to you that an object by itself has no color; instead, its color, as perceived by our eye, is the result of the specific wavelengths of light that the object can mirror or transmit. 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.
Divalenza teaches that to point out the typical color of a gemstone, it is necessary to light it up by a source holding approximately an 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 put out only a small range of wavelengths (e.g., the sodium-vapor lamps used in some cities for street illumination) produce only a portion of the wavelengths settled 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.
According to Divalenza direction, it's not so difficult to understand the color of gemstones. The key for getting this lies in the fact that most substances transmit and reflect by the visible wavelengths of light, some are 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.
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 can identify approximately 128 different hues, ranging from violet through red. Several 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 can tell 150 real hues.
The 150 pure hues Divalenza is talking about, should not be confused with the total number of colors human eye can observe, since any given pure hue can vary from light to dark and from dull to intense. The total number of variations, or coloro, that are noticeable by an 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.