The Metallic Jack of Diamonds are items that can be picked up by Carnby. They were different in terms of appearance but both serve as the purpose of keys to access a locked door.
The first was located under a green chest in the Card Hatch Tunnel, and could be picked up if Carnby pushed the chest. The card can be inserted into the altar in the same room to open up the hatch on the opposite to it.
The second, which resembles a jig-saw-piece can be obtained after defaeating T-Bone the second time on The Flying Dutchman. This key can be used to open the Captain's Cabin.
Appearance[]
The first one resembles a flat playing card, although that would be easy to hold underneath the chest it was under. The second look like a puzzlepiece.