When it comes to fossils, ammolites might just be the prettiest ones around. Ammolites are biogenic gemstones, meaning gems that come directly from a living (or, once living) thing. Specifically, they’re fossilized shells of ammonites, mostly from the genus Placenticeras. Their shells are made of many layers of aragonite, and the thickness of the layers determines the ammolite’s color. Thick layers shine in red and green, and thinner ones reflect violet and blue hues. And when we look for ammolites, we sometimes find bonus fossils, too. The mine where we got these ammolites sometimes also yields mosasaur fossils, which makes sense, given that ammonites were some of mosasaurs’ favorite snacks way back when.  DETAILS You'll receive one hand-selected piece of polished iridescent ammolite Various sizes from 1.5 inches to 2.5 inches across (65mm to 45mm) No subscription required, first come first serve A SciShow Rocks Box find!