🌈 Rainbow Ammonite Fossils – Ancient Light, Eternal Color Each of these stunning ammonite fossils is a relic of primordial oceans, formed over 65 to 400 million years ago, when these now-extinct marine mollusks spiraled through ancient seas. But what makes these particular fossils so enchanting is the natural rainbow iridescence shimmering across their fossilized shells — a phenomenon known as ammolite. This breathtaking play of color is caused by aragonite, a mineral that preserved the ammonite’s shell structure through millennia of heat, pressure, and mineralization. Under just the right geological conditions, the aragonite layers refract light like a prism, creating the dazzling reds, greens, golds, and blues that make each rainbow ammonite uniquely radiant.