The shells and hard structures built by many oceanic organisms are made up of calcite and aragonite crystals! Modern mollusks, oysters, and corals are all examples of this. Extinct species, like this ammonite also used these crystal structures, occasionally during the fossilization process ammonite shells can form a crystal called ammolite. As can be seen in the pictures below ammolite has a very pretty rainbowy iridescence, that was incredibly fun to paint!!
