A real fossil ammonite has been cut in half and then polished, so that you can see the internal structure of chambers. This ammonite comes from Madagascar and is discreetly and permanently attached to a gold coloured post. I think that this one can work in either 1:12 or 1:24 scale.
Ammonites are extinct relatives of modern day nautiloids and squid. The largest known ammonite in the world would have been 8 1/2 feet (2.59 metres) across if it were complete.
Just under 1/2 inch (1.3 cm) wide, 13/16 inch (2.0 cm) tall.