Iridescent ammonite (Placenticeras sp.) Shell, ca. 83.5 million years old, Canada Mineralized ammonite in iridescent aragonite 36x30x4.5 cm Provenance: market (Italy) Conservation status. Surface area: 80%. Conservation status. Support: 80% (gaps, recomposed parts; consolidation using special resins and UV treatment) Extremely rare specimen. The fossil dates back to the Upper Cretaceous, Campanian stage. This special mineralisation occurs thanks to the numerous layers of volcanic ash (bentonite), present only in the Bearpaw Formation in southern Alberta in Canada near the St. Mary River, where the fossil was excavated. In 1981, the World Jewellery Confederation (CIBJO) and the Gemmological Institute of America awarded ammonites mineralised in this way - known as ammolites - the status of gemstones. The weight is in fact measured in carats, as for diamonds, but ammolites are much rarer: in fact, in one tonne of removed sedimentary material there is, on average, 0.6 kt of ammolite as opposed to 1.5 kt of diamond. The distinctive iridescent colours given off by this shell are due to the refraction of light on the different layers of the aragonite shell. Each colour represents a different layer by which light originates an almost complete spectrum of the visible. Very rare even in museums, this mineral fossil is preserved in an exceptional specimen with blue refraction at the Museo Mineralogico Campano.