The Silver Ingot is the forty-sixth treasure that can be found in Uncharted: Drake's Fortune.
The Silver Ingot is found in Chapter 14 - Going Underground. You can find this in the section after viewing the cutscene with the villains. Jump across the gap and drop down off the platform to the other side to get this slab of silver. If you look on the back of it, you will see the Naughty Dog dog paw symbol.