It is actually quite common for gems, gold, and treasure more generally to be passed from kingdom to kingdom and monarch to monarch, often through conquering, warfare, or diplomacy. I am most familiar with the medieval period, and I know that a ruler would often be in possession of very ancient and "exotic" pieces of treasure that functioned to symbolize the legitimacy of their rule and their dominance over the world, either through diplomacy, warfare, or the sheer wealth to purchase such precious objects.

If you went into a royal or national treasury, most of the pieces would certainly be quite old. Because of patterns of conquering and possession, it is quite possible that newer pieces would have been cast out of gold that had been melted down from an original, older piece, perhaps one the nation had conquered. I'm not entirely sure about ancient Egypt specifically, but based on patterns of colonialism (and especially if you look at something like the British Museum as a kind of global vault of imperialism) I'd bet there are several nations that have gold from old and faraway places.