Wouldn't want one that big coming at me.
Few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size exceeding 9 inches in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 2 feet in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest known ammonites, sometimes reaching 6.5 feet in diameter. The largest documented North American ammonite is Parapuzosia bradyi from the Cretaceous, with specimens measuring 4.5 feet in diameter.
Creepy looking in real life
But still iridescent after millions of years. This one's from Madagascar.