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Gemstone Color: Allochromatic & Idiochromatic Coloration



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Gemstone Color, Dispersion and Reflection


The visual appeal and characteristics of a gemstone are determined by several factors including: brilliance (sparkle), color, fire (light dispersion), and luster (surface reflectiveness). A stone's brilliance, fire, and to a lesser extent luster, are influenced by the type of cut used.



The color of a gemstone is due to one or more of several key factors. A mineral can have its own inherent color due to its basic chemical makeup (idiochromatic), or it can start out as a colorless material and gain its characteristic color from trace impurities (allochromatic). A mineral can also display multi-colored characteristics from the scattering or dispersion of light (pseudochromatic) that is reflected from its surface, while having little or no inherent color.


Gemstone Color Caused by Absorption or Reflection

The actual color of a gemstone (not including any optical effects caused by reflection or refraction) is due to the absorptive and reflective properties of any trace elements and impurities within the crystal. Basic elements such as chromium, copper, iron, manganese, and vanadium absorb and/or reflect different wavelengths of the color spectrum playing an integral part in affecting the color of the mineral. Color in gemstones can be caused by chromophores (trace impurities), crystallographic (electronic) distortions in the stone's matrix, or a combination of both.


Saturation of Color

The saturation of color in a gemstone is the amount of light absorbed per some unit path length by the gemstone to produce the color in the gemstone. Highly saturated Elbaites absorb more light and therefor appear less bright than lightly colored Elbaites with the same chromophores assuming that they have similar cuts and are equally well polished. See: Colored Gemstone Grading


Extinction of Light

This is a way to look at the amount of light that is absorbed by the same gemstone such as Elbaite with different chromophores. It can also be discussed under the term tone. It can easily be seen why Elbaite, colored by copper, has a light tone or demonstrates the lack of extinction when illuminated with white light by doing a spectral analysis. Only the very edges of the visible spectrum have a significant amount of light absorption. The production of a rich color through the minimal extinction of light produces a neon sensation in the mind.



When light is reflected off of a gem's surface, some of the visible spectrum is absorbed and some is reflected. When the the green and red components of the visible spectrum are absorbed by a material, only the blue component is reflected back.





Chromophores & "Transition Metal" Impurities

Gemstone color is caused by idiochromatic coloration which is inherent in the chemical makeup of the crystal, allochromatic coloration from the presence of trace elements or impurities within the crystal's chemical makeup, or pseudochromatic color that is caused by surface, or subsurface reflective optics properties to the stone. Idiochromatic minerals are "self-colored", owing their color to chromophores or major constituents in their chemical formula.


Idiochromatic, Allochromatic, & Pseudochromatic Coloration

A chromophore is either an element or physical structure in a substance that produces color. copper bearing tourmaline contains trace amounts of copper. A trace amount is a level of impurity that does not effect a mineral's physical properties, or its color.

1. Idiochromatic Coloration in Gemstones

Minerals that are "self-colored" from major chemical constituents (chromophores) that are key components in their physical makeup.

2. Allochromatic Coloration in Gemstones

Allochromatic coloration that is caused by chromophores from the following "transition metal" trace impurities found within crystalline structures.


Transition Metal Impurities
Transition Metal Impurities
  1. Beryllium (Be): Emerald (blue-green)
  2. Chromium (Cr): Emerald, Jade, Tourmaline (green); Alexandrite, Ruby, Spinel, Topaz
  3. Copper (Cu): Para’ba (greenish-blue), Turquoise (greenish-blue), Malachite (green)
  4. Iron (Fe): Aquamarine, Tourmaline (green); Chrysoberyl, Citrine, Jade (green-yellow-brown)
  5. Lithium (Li): Tourmaline (green, pink)
  6. Manganese (Mn): Tourmaline (pink), Morganite
  7. Nickel (Ni): Opal (green)
  8. Nitrogen (N): Diamond (yellow)
  9. Sulfur (S): Lapis Lazuli
  10. Titanium (Ti): Sapphire (blue)
  11. Vanadium (V): Emerald, Alexandrite, Colored Sapphire (green-red), Tourmaline (green)

3. Pseudochromatic Coloration - Dispersion

Pseudochromatic coloration is the appearance of "color" that is not caused by any actual color in the mineral, but from varying optics effects created by spectral dispersion and refraction.

  1. Fire: Diamond, Zircon

4. Pseudochromatic Coloration - Scattering

False coloration caused from optics effects created by light scattering that is generated by the physical structure of the mineral.

  1. Adularescence: Moonstone (blue), Opal (milky white)
  2. Asterism: Star Ruby, Star Sapphire
  3. Aventurescence: Sunstone, Aventurine Quartz
  4. Chatoyancy: Alexandrite, Tiger's Eye
  5. Defraction (opalescence): Agate, Opal
  6. Flash (Play of Color): Opal
  7. Fluorescence: Diamond
  8. iridescence (schiller effect): Labradorite
  9. Luster (pearlescence): Pearl, Talc, Gypsum
  10. Pleochroism (color change effect): Alexandrite, Rubellite

Pleochroism, Dichroism & Trichroism


See: Pleochroism, Dichroism & Trichroism


Dichromatism

Dichromatism (aka 'polychromatism') is a phenomenon where the hue of the color of a material is dependent on both the concentration of the absorbing substance and the depth or thickness of the medium traversed. An example of dichromatism in a mineral is ametrine, which creates an amethyst-citrine (purple to yellow) coloration in quartz.

The "color-change" effect in stones such as alexandrite is primarily due to any changes in the color of refracted incident light. A single specimen of alexandrite will appear to be a different color under incandescent, fluorescent, and natural sunlight. When alexandrite is viewed under a light source that contains strong red wavelengths (incandescent), the stone will appear red, and when viewed under a light source that contains strong blue wavelengths (fluorescent), the stone will appear blue-green. When the stone is viewed under a light source that containing all wavelengths of the spectrum (natural sunlight), the stone will transmit both blue and red, appearing a purplish-grey.


Photochromism

Photochromism, also known as tenebrescence, is the ability of a mineral to change color when exposed to sunlight. The photochromism is temporary, changing back to the original color when the exposure is discontinued. The photochromism effect is used in the manufacturing of so-called "self-adjusting" or "self-tinting" sunglasses.


Color Orientation

Gemstone crystals rarely have a uniform and consistent hue and color saturation throughout the material. Crystals are natural formations that are subject to many variables in pressure, temperature, and chemical concentrations that can cause irregularities, color banding, and zoning as the material crystalizes. The color in a crystal can also be directional as is the case with tanzanite which can have a brown hue when observing the stone's c-axis, and a blue hue when looking at its a-axis.






Play of Color

The term "play of color" (POC) refers to an optical phenomenon that is similar to iridescence, causing incident light rays to be scattered and broken into their component colors by selective wavelength attenuation. This creates a "rainbow effect" that is also described as "iridescence," which comes from the Greek word iris (pl. irides), meaning "rainbow." As the angle of view changes, so do the colors that are reflected from the object's surface.

The "colors" created by this phenomenon are a form of pseudochromatic coloration, and any reference that is made to them when describing an iridescent material such as opal does not refer to the "body color" (actual color) of the material. This type of coloration is usually described as a color "flash," which describes a particular color that will only be visible when the material is oriented at a certain angle relative to the incident light.


Play of Color and Iridescence in Gemstones

The color flash occurs when some of the incident light is reflected from the material's surface, while some of the light is reflected back to the viewer after it travels though the material and is reflected off of its backside. This latter ray of light is refracted and split due to the different indices of the film and the surrounding air, and this phase shift alters the wavelength of the light ray, therby changing its color.

This phenomenon can be observed in everything from the "thin film" interference of a soap bubble, to seashell nacre, oil slicks, butterfly wings, and minerals such as ammolite, cat's eye, feldspar, labradorite and opal.







Perceived Color & Metamerism

Materials do not actually contain color, but they do have a perceived color when placed under different light sources, which is a result of the light that is returned or extinguished. The difference or similarity of a material's color under different light sources is referred to as "metamerism." In the study of color perception, or "colorimetry," metamerism refers to the matching of the "apparent" color of materials that have different spectral power distributions. Spectral power distribution describes the proportion of total light that is emitted, transmitted, or reflected by a material at every visible wavelength.

The human eye contains only three color receptors called "cone cells," therefore all visible/perceived colors are reduced to three sensory quantities, called "tristimulus values." Metamerism occurs because each type of cone responds to the cumulative energy from a broad range of wavelengths, so that different combinations of light across all wavelengths can produce an equivalent receptor response and the same tristimulus values or color sensation. Metamerism is measured using the Color Rendering Index (below), which is a linear function of the mean Euclidean distance between the test and reference spectral reflectance vectors in the CIE 1964 color space.


Perceived Color & Metamerism

The diagram above shows the difference between "additive color" which is generated by a light, and "subtractive color" which is reflected. Additive Color is the result of visible light emanating directly from a light source, and "subtractive color" is the result of light that is reflected off of a surface. Notice that when you combine Red, Green, and Blue in the Additive Color process, the combination produces white light. In the Additive Color process, white is the combination of all colors. With subtractive color, when you combine Yellow, Cyan, and Magenta, the combination produces black. In the Subtractive Color process, black is the combination of all colors. Your monitor uses Additive Color (RGB), and a printed page uses Subtractive Color (CMYK).






Evaluating Gem Color

When evaluating a stone's color it is important to view comparisons under a light source with a uniform color-temperature, preferably at 5,000K daylight. This is the equivilant of northerly light at midday - without direct sunlight hitting the stone.





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Bibliography & Suggestions for Further Study on Gemstone Color


ICA, All About Colored Gemstones . The Colored Gem Association

UC Berkeley, Color in Minerals . Berkeley.edu

Kurt Nassau, The Origins of Color in Minerals . American Mineralogist

Sarin, Color Grading & Gemology Tools . Sarin Gem Labs

Martin D. Haske, Measuring Color Via Spectrophotometer . AGL Adamas Gemological Laboratory

T. Loomis, P Stevens, Vibrational spectroscopy of minerals . Queensland University of Technology


Bibliography & Suggestions for Further Study on Gemstones

Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition

Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press

Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press






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