Baculite specimen in matrix, displaying red and green iridescence
Photo credit: Frances Goodman
“Colourful” may not be the word that comes to mind when you think about fossils, but some ammonite fossils absolutely are that! This is due to the presence of the mineral aragonite in the ammonite’s shell. Aragonite is composed of many tiny layers, and when light bounces off of these layers, it produces the colours seen in the fossils above! The thicker the layers of the aragonite, the more reds and greens you’ll see; the thinner the layers, the more blues and violets you’ll be able to spot.
When polished, these shells can be of gemstone quality, and are known as ammolite.