Ammolite is a rare and valuable opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is arguably the rarest gemstone on earth, rivaling the rarity of such gemstones as alexandrite and red diamonds. It is made of the opalized portion of fossilized ammonite shells. Ammonites are 350 million year old fossilized remains of an extinct relative of the Nautilus.