A pyritised ammonite is a fossilised shell of an extinct marine mollusk, preserved through a process where the original organic material has been replaced by pyrite.  This mineral replacement gives the fossil a striking metallic gold or brassy sheen.

Over millions of years, these minerals replace the shell or fill its chambers, resulting in a beautifully detailed and often glittering fossil. Pyritised ammonites are commonly discovered in parts of the UK, Germany, and Russia.