The Gold Mosaic Inca Earring is the twenty-first treasure that can be found in Uncharted: Drake's Fortune.
The Gold Mosaic Inca Earring is found in Chapter 5 - The Fortress. This treasure is between a large pile of rubble and an arch-shaped iron gate. You can see where it is if you stand where the Strange Relic is. After getting the last treasure, head down to the section where this treasure is (the section before the machine gunner). You'll have to kill 2 pirates here. Climb over the pile of rubble and grab the earring.