Why are things colored?

Six blue gemstones with different causes of color Clockwise from top left: Maxixe-type beryl, radiation-induced color center (energy bands). Round blue spinel, transition metal impurity (ligand-field color from a cobalt impurity). Spinel "doublet," colorless spinel containing a layer of organic dye - color caused by organic compounds (molecular orbitals). Oval lapis lazuli, S3- anion-anion charge transfer (molecular orbitals). Blue sapphire, Fe-Ti intervalence charge transfer (molecular orbitals). 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.