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 contained in nacre, with a microstructure inherited from the shell. Ammonites were a squid-like marine animal and existed through out the Paleozoic Era until the end of the Crustaceous Era, when they abruptly became extinct.