The opal like iridescence of these ammonites from Alberta, Canada has been termed ammolite and since 1981 has had the status of a gemstone. The shimmering, metallic colours were caused by the combination of millions of years of compression and the mineralization of iron, copper and silica which precipitated from volcanic ash. The opalescent layers of colour are particularly prized in the Chinese philosophical system of Feng-Shui.
The ammonites are a group of molluscs that lived in the seas from the mid-Devonian (approximately 400 million years ago) to the end of the Cretaceous (65 Mya). The sequence of events that famously killed off the dinosaurs also drove the ammonites to extinction