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What is ammolite? http://en.wikipedia.org/wiki/Ammolite
Ammolite is a rare and valuable opal-like organic gemstone, made of the fossilized shells of ammonites (an extinct shellfish whose shells resembled those of the modern chambered nautilus) which in turn are composed primarily of aragonite, the same mineral that makes up nacreous pearls. It is one of several biogenic gemstones; others include pearl, jet and amber. An iridescent opal-like pleochroism is shown in fine ammolite specimens, mostly in shades of green and red; all the spectral colors are possible, however. The iridescence is due to the microstructure of the aragonite: the play of color in ammolite comes from interference with the light that rebounds from stacked layers of thin platelets that make up the aragonite.