Ammolite

Ammolite is a rare organic gemstone with a brilliant, opal-like play of colour. It is not a mineral species in the strict sense but the fossilised, colour-shifting shell material of ammonites — extinct marine molluscs that lived in a warm inland sea over 70 million years ago.

How ammolite forms

The ammonite's shell was made of aragonite, a form of calcium carbonate. Where the original aragonite layers survive in the fossil, light bouncing between these microscopically thin, fractured layers produces flashing reds, greens, golds and blues — the same structural colour seen in opal and mother-of-pearl. Only about 3–5% of ammonite fossils carry gem-quality colour.

Where it is found

Gem ammolite comes almost exclusively from the Bearpaw Formation of southern Alberta, Canada — the only commercial source in the world. In 2022 it was named the official gemstone of Alberta.

For collectors

Bright, multi-coloured sheets with broad colour play are the most prized, as natural fossil specimens or as stabilised polished gems. Because the aragonite layer is thin and delicate, ammolite is often protected under a cap of clear quartz or sold as a triplet.