A page from the "Causes of Color" exhibit...

Why are things colored?


Six blue gemstones with different causes of color

Clockwise from top left:

  1. Maxixe-type beryl, radiation-induced color center (energy bands).
  2. Round blue spinel, transition metal impurity (ligand-field color from a cobalt impurity).
  3. Spinel "doublet," colorless spinel containing a layer of organic dye - color caused by organic compounds (molecular orbitals).
  4. Oval lapis lazuli, S3- anion-anion charge transfer (molecular orbitals).
  5. Blue sapphire, Fe-Ti intervalence charge transfer (molecular orbitals).
  6. Oval shattuckite, copper compound, transition metal compound (idiochromatic ligand-field color).

Scale: the largest stone is 2 cm across

Why is the sky blue? Why is fire yellow? What causes the vivid colors in sapphires or emeralds? What do diamonds and rainbows have in common? Is there a connection between fall colors and flamingos?

It has been proposed that all the colors in the universe originate from a mere fifteen fundamental physical causes. These causes appear over and over, lending color to the world around us. Some common causes seem logical - for example, both light bulbs and candles are colored by incandescence - and others are surprising - did you know that the colors of peacock feathers and bubbles are both caused by interference? Very small additions of an impurity can transform transparent corundum into a gorgeous red ruby. The same mechanism makes emeralds green and produces the color in some glazes and enamels.