What Causes Color in Gemstones?
Color is defined as the visual sensations produced upon the retina by light waves of different lengths.
Light is defined as a form of energy which is radiated by means of electromagnetic waves measured in centimeters or nanometers, which are equal to one-millionth of a millimeter.
Is composed of an approximately equal mixture of all colors or wavelengths that make up the visible spectrum.
A Colored Gemstone in White Light
The color we see is the result of the absorption of the stone of various wavelengths of the original white light.
Transparent Stones – absorption occurs as the light passes through the stone.
Opaque Stones – absorption occurs as the light is reflected from the stone surface.
Selective Absorption: The suppression of certain wavelengths or colors in white light. It is caused either by impurities present in the gemstone (i.e Chromium in Ruby or Iron in Amethyst) or by chemicals in the stone composition (i.e. Copper in Malachite or Manganese in Rhodonite).
Allochromatic Gemstones: Gemstones whose colors are caused by impurities.
Idiochromatic Gemstones: Gemstones who owe their color to their own chemical composition.
Selective absorption of light in both Allochromatic and Idiochromatic gems is caused mainly by the presence of “Transition Elements”.
|Vanadium||Synthetic Corundum (Alexandrite Color change), Blue/Violet Sapphire|
|Chromium||Ruby, Emerald, Alexandrite, Red Spinel, Jadeite, Demantoid Garnet, Pyrope Garnet, Pink Topaz|
|Iron||Amethyst, Sapphire, Peridot, Aquamarine, Tourmaline, Almandine Garnet|
|Nickel||Chrysoprase Quartz, Synthetic Green and Yellow Sapphires|
|Manganese||Rhodochrosite, Rhodonite, Spessartite Garnet, Rose Quartz|
|Copper||Malachite, Turquoise, Synthetic Green Sapphire|
|Cobalt||Synthetic Blue Spinel, Blue Synthetic Quartz, Cobalt Glass and Natural Blue Spinel|
Metamerism: Color change effect seen when a stone moves from one type of lighting to another (i.e Alexandrite).
In Alexandrite, there is a broad absorption band in the yellow part of the spectrum.
Alexandrite appears green by daylight since this light is rich in shorter wavelengths and red in artificial light (not fluorescent lighting) since this light is rich in longer wavelengths.
The tungsten lamp is blue-deficient hence the red color seen in Alexandrite
The phenomenon termed “Selective Absorption” can be made visible by using an instrument called a Spectroscope. By using a series of prisms or diffraction grating, it is possible to analyze the light as it passes through a gemstone. The result is called an “Absorption Spectrum” in which the colors or wavelengths absorbed by the gemstone appear as dark bands.
Prism Type Spectroscope
- Adjust the slit so that the resolution is pertinent to the spectrum being analyzed.
- Immerse the stone in cold water to cool it down.
- Use a strong, cool, concentrated light source. Fiber Optic is recommended.
- Either direct the light through the gemstone (in the case of a transparent stone) or reflect it from the surface (in the case of opaque stones).
- Position the spectroscope in such a way as to receive the transmitted light through the slit.
- Unpolished stones.
- To identify treated stones.
- Faceted stones that have a refractive index above the normal range of the refractometer.
- Identify some synthetics (i.e Natural Blue Sapphire from its synthetic counterpart)
- Wavelengths are not linearly spaced out. The red end is bunched whilst the blue/violet is spread out.
- Utilizes a diffraction grating to disperse the light into the spectral colors.
- The diffraction grating consists of a glass plate onto which a series of fine parallel lines have been photographically printed in the region of 15,000 to 30,000 per inch. It produces a series of diffracted beams which appear as an evenly spaced out spectrum.
- Use in a similar fashion to the prism type spectroscope.
Same as the prism type spectroscope.
- Spectrum is not as bright.
- Hard to regulate the amount of light that enters the instrument.
- Hard to view in the blue end of the spectrum.
- Cost, they are relatively inexpensive.
- Extremely portable.