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 million years ago). The sequence of events that famously killed off the dinosaurs also drove the ammonites to extinction.

The present specimen is one of the best examples to be offered by Christie's in recent years, its preserved shell also features extraordinary polychromy across the shell - especially blues and purples - that are usually found only as trace elements in similar specimens. Unlike most examples seen on the market, there is no infill from other animals, and the stabilising varnish gives an elegant matte appearance.