What are the 'Strange Relics' in the Uncharted series?

There is a trophy in each of the Uncharted games called 'Relic Finder'. To earn it you have to locate the 'Strange Relic'. I don't think these relics are mentioned in the stories at all, and I have found the one in #2 and it just shows up like a normal treasure you find. I haven't noticed anything that it changes yet.

So, what is the significance of the "Strange Relics" in each game? and why are they different from the other treasures you collect?

In answers please use spoiler tags if the answer relates to the story.

The company that makes the game, Naughty Dog, is the creator of another beloved PS2 franchise, Jak and Daxter. The "strange relic" is an egg that was a collectable in the Jak and Daxter games. Its inclusion in the Uncharted series is simply the developer providing a literal "easter egg" referencing their past work.

Additionally, they included the strange relic again in Uncharted 3

The "strange relics" are actually a tongue in cheek reference to the Jak and Daxter games - they're actually Precursor Orbs - hence the significance attached to them. Naughty Dog developed both series, so this is just an easter egg. (One might say literally, given the shape of these things...)

More at the J&D wiki and Uncharted wiki.