Large Iridescent Ammonite

Placenticeras costatum

Late Cretaceous Period (approx. 71 million years ago)

Bearpaw Formation, Alberta, Canada

A large, exceptional specimen displaying superb mineralization in ammolite on both faces, with shimmering and iridescent reds, greens, yellows, blues, and purples. The colors shift in hue and intensity as they diffract light from different viewing angles.

Along with amber and pearl, ammolite is one of the world's few biogenic substances. Resembling the inorganic opal, it is found exclusively in the shells of ammonites that have undergone the fossilization process known as permineralization.

The quality of gem ammolite is determined by a number of factors, with the first and most obvious being its number of primary colors—reds and greens are somewhat common, while blues and purples appear much more rarely. Also of great significance is the range of the ammolite's chromatic shift, graded by the way its colors change in hue and intensity as they diffract light when viewed from different angles. Lastly, an ammolite's magnitude of iridescence also impacts its quality and worth, with the finest specimens displaying large and uninterrupted swaths of lustrous, rainbow-like colors.

Ammonites such as this one lived approximately 70 million years ago in the Western Interior Seaway, a large ocean that once cut North America in half from the Arctic Circle to the Gulf of Mexico. 

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