Iridescent Placenticeras meeki ammonite fossil on display at The Museo Civico di Storia Naturale di Milano (Milan Natural History Museum), Italy as of August 2019. It is quite a beautiful fossil and probably one of the most valuable (price-wise) with exception to the dinosaurs on display there.
It dates to the upper Cretaceous Period and was found in Canada. I would estimate the fossil was at least 30 cm in diameter.
What makes the fossil so valuable is that it is composed of a organic gem-like material ammolite. Ammolite is a biogenic gemstone like pearl and amber. The material comes from the ancient shells of cephalopods. The creatures swam in what was known as the Western Interior Seaway which stretched from today's Arctic Ocean, through western Canada and United States and down to the Gulf of Mexico.
