Wednesday, April 22, 2009


The most common cause of color in gemstones is the presence of a small amount of a transition metal ion. These transition metal ions have an incomplete set of 3d electrons. Changes in the energy of these electrons correspond to the energy of visible light. When white light passes through a colored gemstone or is reflected by it, some of the energy of the visible light is absorbed, causing 3d electrons in the transition metal ion to undergo an energy change. The light that is transmitted or reflected appears colored, because those colors corresponding to 3d-electron energy transitions have been absorbed. The table lists several common gemstones, their chemical compositions, colors, and the origins of these colors.

GemFormulaColorOrigin Of Color
RubyAl2O3RedCr3+ replacing Al3+ in octahedral sites
EmeraldBe3Al2(SiO3)6GreenCr3+ replacing Al3+ in octahedral site
AlexandriteAl2BeO4Red/GreenCr3+ replacing Al3+ in octahedral site
GarnetMg3Al2(SiO4)3RedFe2+ replacing Mg2+ in 8-coordinate site
PeridotMg2SiO4Yellow-greenFe2+ replacing Mg2+ in 6-coordinate site
TourmalineNa3Li3Al6(BO3)3(SiO3)6F4PinkMn2+ replacing Li+ and Al3+ in octahedral site
TurquoiseAl6(PO4)4(OH)8A4H2OBlue-greenCu2+ coordinated to 4 OHG and 2 H2O
SapphireAl2O3BlueIntervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
AquamarineBe3Al2(SiO3)6BlueIntervalence transition between Fe2+ and Fe3+ replacing Al3+ in adjacent octahedral sites
DiamondC Colorless pale blue or yellow Colorcenters from nitrogen atoms trapped in crystal

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