Ammolite is an iridescent, multi-colored gemstone found mainly in Canada. Although ammolite appears opal-like, it is an organic gemstone that is the fossilized shell of ammonite, an extinct mollusk. Ammolite is one of the rarest gemstones because the current supply of ammolite is limited and it's estimated that only 5% of the world's ammolite is gem-grade and usable.