|Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral that makes up nacreous pearls. |
It is one of the few biogenic gemstones; others include amber, pearl and coral.
|The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains. This area is known today as the Cretaceous or Western Interior Seaway. As the seas receded, the ammonites were buried by layers of bentonite sediment.|
This sediment preserved the aragonite of their shelled remains, preventing it from converting to calcite.
|An iridescent opal-like play of colour is shown in fine specimens, mostly in shades of green and red; all the spectral colors are possible, however. |
Iridescence is due to the microstructure of the aragonite: unlike most other gems, whose colors come from light absorption, the iridescent color of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets that make up the aragonite.
|The thicker the layers, the more reds and greens are produced; the thinner the layers, the more blues and violets.|