Massive & Rare Iridescent Fossil Ammonite, Caloceras johnstoni, from England

SPECIES: Caloceras johnstoni

AGE: Lower Jurassic (198-201 million years) 

This is a brilliant example of super rare Caloceras johnstoni ammonite which were free swimming members of the order Ammonitina, super family Psilocerataceae, subfamily Psilococeratinae. Caloceras johnstoni date from the Hettangian - the lowest of the lower lias. They represent one of the earliest forms of Ammonites found in the Jurassic. The preservation of both adult and juvenile specimens suggests that this assemblage is the result of a dramatic environment change.

These thin-walled Ammonites are crushed, as are most of the Ammonites found in this locality. Flattening of the specimen has increased the area of iridescence of the shell, which is composed of alternating layers of aragonite and conchiolin or the minerals that have replaced them during fossilization. These not only transmit light but also diffract it, acting as a Rowland prism to divide light into its component parts while reinforcing certain wave lengths, in this case spectacular greens, reds and yellows (and even some turquoise), much as the prisms do in satellites that study the chemical composition of the stars.

These Jurassic Ammonites are among the most rare in the world. They come from Doniford Bay, Somerset UK and have fossilized in a brilliant metallic ammolite-like finish. It is impossible to capture their true beauty through a photograph, but the stunning colors make an incredible display in any collection. Collecting of these beautiful ammonites is now prohibited and they are sure to increase in value.