A relative newcomer to the world gem market (since the 1960s), Ammolite is a form of aragonite that is obtained from vivid iridescent fossilized ammonite shells mined in Alberta, Canada. The gem material, from the extinct species Placenticeras meeki and P. intercalare, is found only in certain horizons of the Bearpaw Formation of Late Cretaceous age (about 70–75 million years old). Because the iridescent layer is generally thin and fragile, most Ammolite is fashioned into assembled stones. This article describes the history of Ammolite as a gem material and the geologic setting of the main producing mines; offers an explanation for the formation of Ammolite and the origin of its color (i.e., iridescence caused by an interference phenomenon); presents production data, gemological properties, and a grading classification; and describes the manufacturing process.

About the Authors: Keith Mychaluk is a professional geologist in Calgary, Alberta, Canada. Alfred Levinson and Russell Hall are professor emeritus and associate professor, respectively, in the Department of Geology and Geophysics, University of Calgary.