THE COLORS OF GEMSTONES

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.

Gem	Formula	Color	Origin Of Color
Ruby	Al2O3	Red	Cr3+ replacing Al3+ in octahedral sites
Emerald	Be3Al2(SiO3)6	Green	Cr3+ replacing Al3+ in octahedral site
Alexandrite	Al2BeO4	Red/Green	Cr3+ replacing Al3+ in octahedral site
Garnet	Mg3Al2(SiO4)3	Red	Fe2+ replacing Mg2+ in 8-coordinate site
Peridot	Mg2SiO4	Yellow-green	Fe2+ replacing Mg2+ in 6-coordinate site
Tourmaline	Na3Li3Al6(BO3)3(SiO3)6F4	Pink	Mn2+ replacing Li+ and Al3+ in octahedral site
Turquoise	Al6(PO4)4(OH)8A4H2O	Blue-green	Cu2+ coordinated to 4 OHG and 2 H2O
Sapphire	Al2O3	Blue	Intervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
Aquamarine	Be3Al2(SiO3)6	Blue	Intervalence transition between Fe2+ and Fe3+ replacing Al3+ in adjacent octahedral sites
Diamond	C	Colorless pale blue or yellow Color	centers from nitrogen atoms trapped in crystal