The irridescent opal like effect is produced by a phenomenon called thin film interference, which we discussed in our main post on one of the world’s more spectacular gems available athttp://tinyurl.com/pstqvhx. Thin layers from broken fossil ammonite shells are set in between layers of transparent quartz to protect the fragile fossilised nacre, similar to the coating on pearls. Whole fossils like this one are sold entire, and one needs a license to export them from Canada, where the world’s only supply of decent quality material is found. The live animals must have been quite spectacular if they shimmered like this, or even just like their distant relatives, the nautili.
Image credit: Jewel Expert