Wristwatch Shield

Percy's sheild is a sheild given to Percy by Tyson.

What is it.
It is a shield that is encased within a watch. It has a series of engravings on it depicting the events that took place during the Sea of Monsters. It saves Percy's life many times, and gets damaged once by the Manticore. Percy loses it forever when he throws it at Kampe to keep her away from Tyson long enough for them to escape.

Magical properties
it can transform into a watch Since it was one of Tyson's first projects, and wasn't when he was in Poesidon's forges, it most likely was just a normal shield that could transform into a watch