The Decorated Gold Ring is the third treasure that can be found in Uncharted: Drake's Fortune.
The Ring is found in Chapter 2 - The Search for El Dorado. The treasure is located in th area where you have to shoot a barrel in order to topple a pillar. The treasure is on the lower side (opposite the barrel and pillar). Face the barrel and pillar and move left along the ledge, to the side with the wooden scaffolding. The treasure is in the very corner, between the pillar and the edge of the gap.