The optical characteristic of all gemstones are caused by their chemical composition and crystal structure. Optical characteristics can also be surface, or internal blemishes. Below are some optical characteristics that deal with color:Color of Gemstones:
Color is the result of selective absorption or transmission of different frequencies of visible light. Color is described as the combination of the three following categories: hue, tone, and intensity.
Hue - the frequency of light and is described by familiar terms such as red, orange, yellow, blue, green, indigo, and violet.
Tone - the color variation from very light to very dark.
Intensity - a measure of saturation, or purity, of a color.
The differences between colors may be extremely obvious or so subtle that direct comparison under controlled conditions is needed to differentiate them. Color grading is so highly affected by fatigue, diet, lighting, education and other human factors, so no color grade is an ABSOLUTE. A color grade should be known as "color range".
Color Change in Gemstones:
Changing of colors in gemstones is known as Pleochroism. Pleochroism is the change in color of a refractive gemstone when viewed through different directions of the crystal. In most cases, the color variation is not very obvious to the untrained eye and must be viewed through a polariscope or dichroscope. In many cases, the pleochroic colors are strikingly obvious.
Singly refractive (or isochroic) - the velocity of constant light through all directions in the stone is said to have one refractive index.
Doubly refractive (or anisotropic) - the velocity of light varies with direction through the stone is said to have two refractive indexes. In anisotropic gemstones, light is separated in to two polarized components, the ordinary ray and the extraordinary ray. (can be further characterized as uniaxial, biaxial positive, and biaxial negative).
Amorphous (non-crystalline) material - opal, amber, and glass, may scatter light in unusual directions due to internal characteristics and display a phenomenon known as anomalous double refraction.
Refractive index (or R.I) - the ratio of the velocity of light in air to the velocity of light through a transparent gemstone. Gemstones with high R.I. are more brilliant than those with low R.I.
Birefringence - the difference in value between the highest and lowest R.I in a doubly refractive (anisotropic) material.
Dispersion - the ability of a gemstone to separate light in to its component colors.
Fluorescence - when exposed to ultraviolet light or X-rays and transform some of the incoming energy into visible light.
Phosphorescence - When the gemstone continues to emit light after the UV or X-ray light is removed. This phenomenon usually only will last a few seconds but could, on occasion, last much longer periods and is a rare characteristic in gemstones.
Clarity - will vary from complete opacity to lucid clarity and may contain few or many inclusions such as crystals of other minerals, gas- or liquid-filled cavities, or even insects. (Preserved insect specimens in amber are highly prized.) In some gemstones, such as emerald, certain inclusions are highly distinctive and can be used as reliable indicators of identity.
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