Hello! Thanks for the ask, and sorry for the late reply.
I have some ammolite in my collection. It’s a thin layer on the shells of ammonites, which are mainly formed of aragonite and calcite (over time aragonite turns to calcite and ammonites can even turn to pyrite if conditions are right). It is a opal-like organic gemstone and pretty valuable because of its iridescence and desire for jewellery. I have a good medium sized ammonite which is polished to show off the strong (mostly) red iridescence, and some smaller ones that show a bit of it. As well, because it needs to bounce off of light, it’s better in certain light-filled environments to show the spectrum in the microstructures.
They’re beautiful specimens. I much prefer them in their natural state than in jewellery (unless kept whole) Here are some examples: