Dispersion

Dispersion of Light

Definitions
The divergence or spreading of the different coloured rays of a beam of composite light when refracted by a prism or lens, or when diffracted, so as to produce a spectrum, especially in reference to the amount of this dispersion. (O.E.D.)
The power of a transparent medium to separate the spectrum colours.the figures being usually the result of the subtraction of the figures of refractive index for the red ray ("B" line at 6867Ä.) from the figures of the refractive index for the violet ray ("G" line at 4308Ä.). (Gemmologist's Compendium by Webster)

Dispersion
Dispersion is the difference between the amount of refraction of different colours of light. White light is actually composed of light of all different colours. A highly dispersive material will split light strongly into its component colours to give a "prism" effect showing a "rainbow" or spectrum.
Dispersion is often known as "fire".
Diamond owes most of its beauty to its high dispersion, whereas white sapphire, with low dispersion tends to look "dead" or glassy.

Dispersion of Various Gems and Minerals

Name	Dispersion
Fluor	0.007
Calcite (e-ray)	0.008
Calcite (o-ray)	0.017
Silicon Glass	0.010
Beryllonite	0.010
Leucite	0.010
Cancrinite	0.010
Kyanite	0.011
Pollucite	0.012
Feldspar	0.012
Apatite	0.013
Brazilianite	0.014
Beryl	0.014
Topaz	0.014
Smithsonite (o-ray)	0.014
Smithsonite (e-ray)	0.031
Hambergite	0.015
Phenakite	0.015
Chrysoberyl	0.015
Fibrolite	0.015
Euclase	0.016
Danburite	0.016
Datalite	0.016
Crown Glass	0.016
Andalusite	0.016
Iolite	0.017
Tourmaline	0.017
Spodumene	0.017
Scapolite	0.017
Corundum	0.018
Kornerupine	0.018
Sinhalite	0.018
Idocrase	0.019
Peridot	0.020
Spinel	0.020
Kyanite	0.020
Pyrope Garnet	0.022
Staurolite	0.023
Almandine Garnet	0.027
Hessonite Garnet	0.027
Spessartite Garnet	0.027
Willemite	0.027
Dioptase (o-ray)	0.028
Dioptase (e-ray)	0.036
Epidote	0.030
Zircon	0.038
Flint Glass	0.041
Diamond	0.044
Anglesite	0.044
Benitoite	0.044
Sphene	0.051
Demantoid Garnet	0.057
CZ, C.Z., Cubic Zirconia	0.06
Cassiterite	0.071
Moissanite	0.104
Blende	0.156
Strontium Titanate	0.19
Anatase (o-ray)	0.213
Anatase (e-ray)	0.259
Rutile	0.28

Notes
O-Ray = Ordinary ray
E-Ray = Extraordinary ray, a ray in doubly refractive uniaxial mineral in which the velocity varies according to the direction in which it passes through the crystal.

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Gemstone Characteristics
Hardness - Moh's Scale
Refractive Index
Specific Gravity or Density
A to Z of Gemstones