By
Janice Erickson, G.G.
COLOR
Imagine a world without color, where everything is either black or white or varying shades of gray. No one would be able to feel “blue”, see “red”, or be “green” with envy. There would be no “oranges”, nor would we have a cheery, yellow sun. Now, look around you and see how your world is enhanced by color. My apologies to those who are getting the mailed version of the newsletter. The on-line version is greatly enhanced by the use of color.
If you have the opportunity, please visit the web site or perhaps someone with a color printer will bring a copy in for others to see.What is color, anyway? How do we see color? What causes color? How do we describe color? Color is a complex topic far beyond my expertise, but here is some basic information about color you might find interesting.
Color is the specific response of the eye and associated nervous mechanisms to certain kinds of light. Radiation from the sun consists of a nearly continuous range of wavelengths, from wavelengths of thousands of feet to
wavelengths so short there is no direct means of measurement. Visible light has a wavelength of 1/10,000 of an inch. Visible light is just a small part of the electromagnetic spectrum and has a wavelength range from about
780 nanometers (nm) (or red light) to about 380 nm (violet light). In between the red and violet light are the other colors of the spectrum: orange, yellow, green, blue and indigo. An easy way to remember the order of the colors
of the spectrum is “ROY G. BIV”. Remember that black and white are NOT considered colors. When all the wavelengths of the visible light spectrum strike your eye at the same time, white is perceived. Thus, visible light is
sometimes referred to as white light. Black is merely the absence of the wavelengths of the visible light spectrum.
There are three main factors which determine the color of an object: the object itself, the light in which the object is viewed and the eye and associated mechanisms. We perceive color when the different wavelengths composing white light are selectively interfered with by matter (absorbed, reflected,
refracted, scattered, or diffracted) on their way to our eyes.In the eye
Light from external objects enters the eye through the pupil. The human eye has a lens and iris diaphragm which serve similar functions to the corresponding features of a camera. The optics of the eye form an upside-down image of those objects on the rear, inner surface of the eyeball (the retina). There, a dense carpet of light sensitive photoreceptors converts light (photons) into electro-chemical signals, which are processed by neural circuits in the retina and transmitted to the brain via the optic nerve.
The retina contains two types of photoreceptors, rods and cones. The rods, located in the peripheral retina, give us our night vision, but can not distinguish color. Cones, located in the center of the retina (called the macula), are not much good at night but do let us perceive color during daylight conditions.
Light Absorption, Reflection, and Transmission
When light strikes an object, a number of things could happen. Part of the light wave could be absorbed by the object, in which case its energy is converted to heat; part of the light wave could be reflected by the object; and
part of the light wave could be transmitted by the object. Objects have a tendency to selectively absorb, reflect or transmit light of certain frequencies. One object might reflect green light while absorbing all other frequencies of
visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light.The color of an object is not actually within the object itself; rather, the color is in the light which shines upon it that ultimately becomes reflected or transmitted to our eyes, that is, the residual color. We know that the visible
light spectrum consists of a range of frequencies, each of which corresponds to a specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light which strikes the object and becomes reflected or transmitted to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light which strikes the object and which is not absorbed or transmitted by the object.The light in which an object is viewed affects the perceived color of the object. For instance, the chrysoberyl variety called alexandrite, one of the “color change” gemstones, appears to be a difference color depending on the
light source. When viewed in sunlight, it could appear yellowish, brownish, grayish or bluish green. In incandescent light, it may appear orangey or brownish red to purple–red.Residual Colors in Gemstones
What causes residual colors of gemstones? Do all gemstones of the same “color” have the same cause of color? I found one reference on the internet that pictured six different gemstones of approximately the same hue and they
had six different causes of color.It has been determined that all the colors of the universe are due to only fifteen fundamental physical causes. A gemstone may be “colored” by any of the following causes: 1) idiochromatic or allochromatic transitional elements; 2) color centers; 3) charge-transfer mechanisms between like and unlike ions; 4) organic pigments; 5) conductors; 6) semi-conductors; 7) doped semi-conductors and 8) physical optical properties.
Most gemstones would be colorless if they were chemically pure. The first group of minerals are those whose color is caused by “transitional elements”. The eight transitional elements which have the most impact on color are copper, iron, manganese and chromium (idiochromatic elements) and nickel, cobalt, vanadium and titanium (allochromatic elements). These transitional elements occupy consecutive positions in the periodic table with atomic numbers 22 (titanium) through 29 (copper). If the transitional elements are an essential element, the gem is “idiochromatic” or self-colored. For example, if copper is the transitional element, such as in malachite and tur-quoise, the gem will always have a green/blue color. If the transitional element is an impurity, the gem is “allochromatic” or impurity-colored.
Chromium produces the finest colors in gemstones, from the green in emeralds and uvarovite to the reds in rubies. Green is the most prevalent color due to chromium. Alexandrite is also colored by chromium. Iron can produce gemstones in green (sapphire and peridot), yellow (orthoclase and spodumene), blue (aquamarine and spinel), brown and red (almandine garnet) colorations. Cobalt is usually associated with the color blue, but minerals containing cobalt are pink, not blue. Vanadium is the cause of the bright green of tsavorites. Titanium is part of the coloring of blue sapphires, tanzanite and benitoite. Titanium may also be responsible for the pink in rose quartz. Nickel is the cause of the apple green color of true chrysoprase and green opal from Kenya. Manganese produces a range of pink to reds in the idiochromatic gems of rhodochrosite, rhodonite and spessartite garnet. As an impurity, manganese also colors morganite and pink tourmaline.
The second group of minerals owes its colors to “color centers”, which is the name given to defects in a crystal at which an unpaired electron may be trapped in a location where it would not normally be present. Gemstones colored by color centers include amethyst and smoky quartz, blue topaz and colored zircons.
The third group of color is said to be due to “charge transfer”, which involves a change of valency (transfer of an un-paired electron) between two different transition elements or between two ions of the same element having different
valency states.The fourth group includes organic pigments in amber and coral. The fifth group of minerals include the metals such as gold, copper, and silver in which there is free movement of the outer electrons from atom to the next which makes them good conductors of electricity. The sixth group are the semi-conductors, including diamond and minerals such as galena and pyrites. In these crystals, there is covalent bonding which involves a sharing of electrons between atoms rather than a transfer. The seventh group of minerals include “doped” semi-conducting crystals, such as diamond containing boron atoms. As little as one part of boron per million induces the blue coloration and enables the diamond to conduct electricity. A diamond colored blue by irradiation, however, is due to the formation color centers.
The final group of minerals have their colors due to physical optics, including dispersion, scattering (chatoyancy, asterism and schiller), interference and diffraction (labradorites).
Man has not been satisfied with the natural colors of gemstones and has long sought ways of enhancing the colors found in nature. Color enhancement is another long topic itself and will be left to another article.
Now that we’ve defined color and determined what causes color, how do we describe color?
Color is described in the terms of hue, tone and saturation.
Hue is described as the shade, tint or sensation of a color. The hues are the colors of the spectrum (ROY G. BIV) and ranges of colors in between the main hues, plus purple (which is a combination or red and blue on opposite ends of the spectrum). The GIA GemSet® has 31 Hues which can be used to describe virtually all colored gemstones. The complete GIA GemSet® has 324 sample hue colors with varying Tones and Saturations. If you are going to grade colored stones, it is important to have a standard color reference - color memory is a figment of the imagination.
Tone is described as the relative lightness or darkness of a Hue. The GIA Tone Scale is divided into 11 grades, 0 to 10, with 0 being colorless to 10 being black, with each of the numbers specifically defined.
Saturation is described as the strength or purity of a Hue. The GIA Saturation Scale is from 1 to 6, again with each of the numbers specifically defined. The lower numbers such as 1, 2 or 3 of warm colors such as red, orange or yellow and tend to look brownish and the cool colors such as blue and green tend to look grayish. Level 4 no longer shows either grayishness or brownishness, while neither is strong or weak. Level 5 is strong and level 6 being vivid, almost over-colored.
In the professional world, the Hue, Tone and Saturation of a gemstone is written in abbreviation, using the numerical value for tone, saturation, and the abbreviation for the hue--in that order. For example, an abbreviation of 5.4.vslbG, it translates into English as a Medium Moderately Strong very slightly bluish Green stone.
Each species and variety of gemstone has its own grading grid to evaluate the hue, tone and saturation of any stone of that variety in relationship to the “perfect” color for that variety. The closer the color of your stone to perfect, the more value that stone will have.
The Psychology of Color
How do colors affect our moods? That’s a another story.
And remember, the light in which you view the object influences the “color”!
