Buford on ebay

Buford is an eBay junkie. He bragged to Festus that he bought the following items.

A prehistoric arrowhead made of copper.
The fossil skeleton of a dinosaur no bigger than a chicken.
An ancient Roman coin marked 120 B.C.
A red diamond ring.
An ancient Egyptian cat mummy.

Without even looking at the items, Festus said, "Buford, you big dummy! One of them's a fake for sure."

Which item can Festus positively know is fake?

Give up? . . . check the answer.