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 isreflected 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 thesecolors. A ruby is a crystal of alumina, aluminum oxide, containing a trace of chromium(III) ions replacing some of the aluminum ions. In ruby, each Al3+ ion and Cr3+ ion is surrounded by six oxide ions in an octahedral arrangement. This arrangement splits the five 3d orbitals of Cr3+ into two sets, the dxy, dxz, dyz orbitals and the dx2-y2 and dz2 orbitals. These two sets have different energies.The energy difference between these sets corresponds to the energy of visible light. When white light strikes a ruby, the gem absorbs the light of energy corresponding to the transition of an electron from the lower-energy set of 3d orbitals to the higher-energy set.
Gem Ruby Emerald Alexandrite Garnet Peridot Tourmaline Turquoise Sapphire
Formula Al2O3 Be3Al2(SiO3)6 Al2BeO4 Mg3Al2(SiO4)3Mg2SiO4 Na3Li3Al6(BO3)3(SiO3)6F4 Al6(PO4)4(OH)8A4H2O Al2O3
Color Red Green Red/Green Red Yellow-green Pink Blue-green Blue
Origin of color Cr3+ replacing Al3+ in octahedral sites Cr3+ replacing Al3+ in octahedral site Cr3+ replacing Al3+ in octahedral site Fe2+ replacing Mg2+ in 8coordinate site Fe2+ replacing Mg2+ in 6coordinate site Mn2+ replacing Li+ and Al3+ in octahedral site Cu2+coordinated to 4 OHG and 2 H 2O Intervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites Intervalence transition between Fe2+ and Fe3+ replacing Al3+ in adjacent octahedral sites Color centers from nitrogen atoms trapped in crystal
Colorless, pale blue or yellow
The ruby reflects or transmits the remainder ofthe light. Because this light is deficient in some energies (those that were absorbed), the light appears colored. The origin of the color of emeralds is similar to that of the color of rubies. However, the bulk of an emerald crystal is composed of beryl, beryllium aluminum silicate, instead of the alumina which forms rubies. The color is produced by chromium(III) ions, which replace some of thealuminum ions in the crystal. In emeralds, the Cr3+ is surrounded by six silicate ions, rather than the six oxide ions in ruby. These silicate ions also split the 3d orbitals of Cr3+ into two sets. However, the magnitude of the energy difference between the sets is different from that produced by the oxide ions in ruby. Therefore, the color of emeralds is different from that of ruby. Chromium(III)also produces color in alexandrite. The color of this gem is very unusual, because in bright sunlight it appears green, but in incandescent light it appears red. This unusual behavior is a result of the way human vision works. Our eyes are most sensitive to green light. Alexandrite reflects both green and red light. In bright sunlight, the proportion of green light is greater than it is in thelight from an incandescent lamp. The light reflected by alexandrite in bright sunlight is rich in green light, to which our eyes are most sensitive, and we perceive the gem as green. The light reflected by alexandrite in incandescent light is much richer in red, and we see the stone as red under these conditions. Energy transition of the 3d orbitals of other transition metal ions are responsible...