Emerald and Pearl Necklace is a treasure in Uncharted 3: Drake's Deception. It is found in Chapter 14 - Cruisin' for a Bruisin'.

As you get to the open deck with shipping containers, you must fight a bunch of henchmen. When you get a chance, climb the yellow container in the middle and walk along the top of it into the alcove it created. The treasure will be on the ground.