The Mechanisms of Color Phenomenon
By Paul Hlava
What makes the sky blue?
What causes opalescence?
What causes the play of color?
What causes the moonstone effect?
What causes the color in diamonds and gems?
What causes the fire in diamonds and gems?
(Let's find out!)
Review the nature of light, the perception of color, and the 15 causes of color, 14 of which are caused by electrons interacting with light. Describe color centers, band gap colors, scattering, dispersion, interference, and diffraction.
We know from Color, Part 1 that light is a form of energy, white light is made up of the 7 colors of the solar spectrum. If you remove some color from white light the visible result is the complimentary color. Unpaired electrons cause color by absorbing some colors (energies) of light in order to move into excited energy levels. Color may by caused by transition metal absorption where the electrons in transition metal atoms (often minor contaminants) in minerals and gems do the absorbing. Intervalence charge transfer occurs when an electron can bounce back and forth between different atoms by absorbing some energy from light and thereby cause color.
Color Centers, also known as “farbe” (German for color) or “F” centers, are formed when atoms are oxidized, translated, or removed, usually by radiation, from their normal position in the crystal structure. The resulting hole may be filled by an electron from a neighboring atom. Any unpaired electrons left behind can now be excited and absorb light. Many color centers are unstable and can be destroyed by heat or strong light, such as ultraviolet light.
Many of the colored varieties of quartz (pure Si and O - see notes at end of article if you are not familiar with the chemical notations for the elements) are caused by contaminant elements such as Al and Fe. The colors seen in smoky quartz occur when: an electron in an O atom next to an Al atom is knocked out of orbit by radiation and the unpaired electron left behind in the O site can now absorb light and get excited into higher energy orbitals. Fe3+ should cause quartz to be yellow but the color is changed to amethyst when 1.) the Fe3+ is oxidized by irradiation to the Fe4+ state, 2.) then there is charge transfer of an electron from an adjacent O to the Fe4+ turning it back to the more stable Fe3+, and therefore, 3.) the unpaired electron in the O can now absorb some colors and leave behind the purple. Green diamonds can be caused by losing C atoms due to irradiation, filling the hole with an electron from a C next to the hole, and then having the unpaired electron left behind in the C atom absorb some light energies. Color centers for topaz produce blue and brown colors and occur from two different and unknown color center processes. The brown color in topaz is unstable. The unstable color of Maxixe beryl is related to carbonate radicals in the crystallographic tubes. Amazonite occurs when Pb2- and (OH) are present in microcline feldspar.
Band Gap Colors:
Insulators have a gap in the energies between the electron valence band and the conduction band and thus tend to be transparent. Conductors have no such gap, contain electrons that can absorb all energies of light, and thus tend to be black and opaque. Semi-conductors have intermediate gaps so that electrons can absorb some energy, but not all energies. These tend to be colored. Semiconductors are intrinsic if this color is normal and doped if the color is caused by a contaminant atom in a normally clear, insulating mineral. For example, diamonds doped with N are yellow and those doped with B are blue.
Scattering of Light:
Particles scatter light. Such particles include: dust, molecules, clusters of molecules, random collisions of gas molecules, tiny fat globules, and suspensions. Blue light is scattered more that red light. Lord Rayleigh described scattering of blue and red light by a formula where the intensity of light scattered divided by the intensity of the original light is equal to some constant divided by the wavelength of that light to the 4th power. Using this equation we calculate that if the intensity of blue light scattering is set to 100 the red light scattering in the same situation is only 10.7. Scattering of light in our atmosphere by very tiny dust particles, random collisions of air molecules, even small density gradients causes the sky to be blue. The sun appears red as it goes toward the horizon because the blue has been scattered out. As particles become larger they scatter the other colors better and these colors join with the blue until the color fades to the pure white of clouds, fog, mist, bull quartz, etc.
Examples of scattering effects include: blue skies and red sunsets; milk in water; white clouds; fog and mist; blue eyes in babies; blue veins and arteries under the skin; milky color (opalescence - see note below) of opal, milk glass, and quartz; cat's eye stones; star stones; moonstones.
Scattering/Cat's-eyes or Chatoyancy:
Chatoyancy is an optical effect displayed by certain gemstones cut en cabochon. Very narrow linear features in the stones scatter light in a plane perpendicular to their length, producing cat's-eyes. These linear features may be needles of another mineral, such as rutile in chrysoberyl, or the tubes/voids sometimes found in aquamarine and tourmaline. A thin bright line is produced across the stone that resembles a cat's eye. The sharpness of the cat's-eye depends upon the density of the needles, their fineness, the quality of their orientation with the symmetry of the crystal, and the roundness of the cabochon.
Scattering/Star Stones or Asterism:
When multiple orientations of linear features scatter light in planes perpendicular to each set of features, the effect produces star stones. The included needles may be oriented along two or more crystal planes. When light hits these inclusions, it creates two or more bands of reflected light or eyes. When the stones are cut en cabochon, these bright bands intersect at the apex of the cabochon dome and form stars. The effect is also known as asterism. Stones from the hexagonal crystal system exhibit a six-rayed (rarely a 12-rayed star) star include ruby, sapphire, beryl, and quartz. Garnets form in the cubic crystal system and can sometimes show a four-rayed star.
Dispersion or Fire:
In 1666, Newton used a prism to separate or disperse the white light from the sun into the component colors of the spectrum. These colors are red, orange, yellow, blue, green, indigo, and violet. Fire in gemstones is the result of their ability to disperse the various colors of the spectrum just like the glass prism. Faceted gemstones therefore act as complicated prisms that disperse light into colors. Because different colors come out different facets the result is called fire.
Interference colors are produced when a light wave splits upon entering a transparent medium, the various colors suffer differing retardation, and thus some are out of phase when they try to recombine outside of the medium. Interference often occurs when light rays are split at interfaces. Part of the light is reflected back at the interface, and part of the light is refracted downward. Because the speed of light is less in matter than in a vacuum, the refracted light is retarded. The various colors are dispersed by the medium and so each color follows a different path and is retarded to a different degree. Upon reaching another interface, some of the refracted light is reflected back and eventually exits the medium and tries to recombine with the original reflected part. Constructive interference causes color to be amplified. In this case, light waves traveling the longer path are retarded by an integral number of wavelengths and so they exit in phase, or in step, with the original reflected part. As a result, they reinforce each other and intensify the color. Destructive interference causes colors to be destroyed. Here, light waves on the long path travel so many integral wavelengths and a fraction more. Some are exactly out of phase with the original reflected light and, consequently, destroy each other. Low interference orders (reflected from the top and first, second, etc. interfaces) exhibit brilliant hues while high orders (5th, 6th, etc.) show pale colors.
The play of color in labradorite, oil slicks on water, tight stacks of glass or cellophane, thin cracks in minerals, etc. are all examples of interference colors. This is also known as an iridescence and can be seen in fire agate, the wings of some butterflies and the feathers of some birds.
Diffraction is a special case of interference caused by perfectly aligned layers of identically sized spheres of hydrous silica. Diffraction occurs when light waves bend as they pass the edge of an object. Light waves fan out (disperse) and bend through narrow openings between spheres, and they overlap and try to coalesce when there is more than one opening between spheres giving rise to interference.
The origin of the marvelous “play of color” in opal remained a mystery
until 1965, when the scanning electron microscope revealed its cause. Two
Australian gemologists, Darragh and Sanders, published The Structure of
Precious Opal. Their research explained how patterns of spheres of silica
interspersed with water caused color in opal. The color depends upon the
diameter of the spheres of silica, the uniformity of the spheres and their
alignment, and the angle of incident light. This combination of diffraction
and interference causes color in opals. The interference caused by the
overlapping of light waves reinforces some colors and cancels others.
This effect also shows how precious opal differs from potch. The perfect layers of identical spheres act like layers in normal interference, but different orientations give rise to differences in spacings yielding different colors. Scattered light from spheres of irregular size, shape, and/or alignment causes the milkiness or opalescence seen in potch.
Colors found in gems and minerals can be caused by many mechanisms, and most (fourteen out of fifteen) of these involve electrons. Many of the important causes of color involve absorption of some energies of light. Some of the mechanisms that cause the most spectacular colors are the result of physical phenomena. The origin of color in many gems and minerals is still not completely known, nor fully understood. Much research remains to be done and is being done.
Chemical Notations - Al=aluminum, B=boron, C=carbon, Fe=iron (3+ and 4+ are valence states), N=nitrogen, O=oxygen, and Si=silicon. The purists in the audience may note that I use the term atom when I actually should use ion. I chose to use the one term for both so as to not confuse the less technically oriented readers
Opalescence - There seems to be some confusion as to the meaning of opalescence. Ordinary dictionaries incorrectly ascribe the name to the “iridescence” or “milky iridescence” of opal. Authorities in the field (Dana, Nassau, Downing, etc.) state that opalescence is the milkiness only.