Psiloceras planorbis
Early Jurassic
Watchet, Somerset, England

This thin-walled ammonite is crushed, as are most of the ammonites found in this locality. Flattening of the specimen has increased the area of iridescence of the shall, 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 blue, much as the prisms do in satellites that study the chemical composition of the stars.
Copyright © 2000 Giraud Foster & Norman Barker