Why Do Ammonite Fossils Have Iridescence?

Ammonite fossils have been found all over the world, in different forms of mineralization.  Why do some have a lovely iridescence? 

The ammonite creature, while alive, produced an organic layer of nacre on the shell.

This organic material fossilizes over time into aragonite. The layers are different thicknesses. 

When light hits the material, it enters and filters through the different layers of the aragonite.  Some of the colored wavelengths of light are absorbed and others are reflected back.  The colors of light that are reflected back are the colors that we end up seeing.

Thick layers of this aragonite cause the green through red wavelengths to be reflected back. Many aragonites demonstrate these colors.

Where the layers of aragonite are thin, the purple and blue ends of the light spectrum are reflected back.  Because these thinner layers are more susceptible to damage and decay, it is more rare to find an ammonite that reflects purple and blue iridescence.

These fossilized remains of ancient sea creatures make a wonderful addition to any crystal, mineral or fossil collection-no matter what colors of the rainbow we see!