This fossil ammonite has been cut in half and then polished, so that you can see the internal structure of chambers, now filled with minerals. Sometimes an area of a different colour, as at the base of this one, is because the shell there was open to the sea initially, perhaps due to damage, and so mud entered and fossilized a different colour from the rest of the fossil. This ammonite comes from Madagascar and is discreetly and permanently attached to a gold coloured base.
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.
1 inch (2.5 cm) wide, 5/16 inch (0.8 cm) deep and 1 1/8 inches (2.9 cm) tall.