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
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, iron, manganese, and vanadium absorb or reflect different wavelengths of the color spectrum playing an integral part in affecting the color of the mineral.
Color Absorption & Reflection in Gemstones
The diagram above shows the difference between Additive Color (generated) and Subtractive Color (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. In the Subtractive Color process, when you combine Yellow, Cyan, and Magenta, the combination produces black. In the Subtractive Color process, black is the combination of all colors.
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. Your monitor uses Additive Color (RGB), and a printed page uses Subtractive Color (CMYK).
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.
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.
- Beryllium (Be): Emerald (blue-green)
- Chromium (Cr): Emerald, Jade, Tourmaline (green); Alexandrite (green), Ruby, Spinel, Topaz
- Copper (Cu): Paraiba Tourmaline (blue), Turquoise (blue), Malachite (green)
- Iron (Fe): Aquamarine, Tourmaline (green); Chrysoberyl, Citrine, Jade (green-yellow-brown)
- Lithium (Li): Tourmaline (green, pink)
- Manganese (Mn): Tourmaline (pink), Morganite
- Nickel (Ni): Opal (green)
- Nitrogen (N): Diamond (yellow)
- Sulfur (S): Lapis Lazuli
- Titanium (Ti): Sapphire (blue)
- Vanadium (V): Emerald, Alexandrite, Colored Sapphire (green-red)
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.
4. Pseudochromatic Coloration - Scattering
False coloration caused from optics effects created by light scattering that is generated by the physical structure of the mineral.
The pleochroic color-change effect in Alexandrite is primarily due to any changes in the color of 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.
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.
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; 3rd edition
Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press
