Unusual Mask Brooch is a treasure in Uncharted 3: Drake's Deception. It is found in Chapter 14 - Cruisin' for a Bruisin'.
Just after jumping from the burning boat to the cruise ship at the start of the level, you will run toward the camera. The treasure will be on Nate's left, and is clearly visible when you run past it. It will be on the ground in a small alcove.