Ammolite

Ammolite is an opal-like organic gemstone formed from the fossilized shells of ammonites, which were marine mollusks that lived millions of years ago. It is composed primarily of aragonite, the same mineral that makes up the outer shells of modern-day mollusks such as snails and clams. Ammolite is known for its stunning array of iridescent colors, which are caused by the diffraction of light as it passes through the thin layers of aragonite.