Ammolite is both fossil and gemstone. It comes from the aragonite mother-of-pearl surrounding ammonites – jokingly called “grandmother-pearl”. Many ammonites still exhibit an iridescent sheen to them, however the ones from mines in southern Alberta exhibit a rainbow spectrum of gem-quality colors. Aragonite in life is an unstable organic crystal form of carbonate, and generally when a shell fossilizes, as it’s already a mineral, simply reverts to a more stable form of calcite. With Ammolite, the aragonite layers haven’t reverted – and the more layers there are, the more colors are refracted.
It was given gem status in 1981 by the World Jewelry Confederation. It is one of three organic gemstones – the other two being amber and pearls. If used in jewelry, because of the soft nature of the stone (only a 3.5-4 on the Mohs scale), it should only be set in earrings or pendants – not rings.