This is a lovely piece of fossil jewellery. The Orthoceras was a type of prehistoric squid and  lived between 400 and 500 million years ago.
This Orthoceras showing in the black matrix has been skilfully carved into a bullet shape measuring  roughly 4.5cm long - then strung from an attractive 18 inch solid silver curb chain along with a little polished light green Amazonite bead.

The pendant you receive won't be the exact one pictured, but very similar. These are natural objects so the fossil size and siting on the pendant may vary slightly but the basic size of the pendant will be pretty consistent.

This is a lovely item and very unusual!