LotR -- Book 1 - Chapter 12 - Flight to the Ford

[Firefoot]

The green "elf-stone" that he left on the bridge must have been the same kind of jewel as the Elessar then ?

I wondered about this as well, and I am pretty sure that they are of the same type. I can see why even if they weren't the same Aragorn still might take it as a sign, being Elvish and all, but I think it probably had special significance to Aragorn because it was foretold that he would take the name of Elessar. Perhaps this was why he was able to take it for a sign that he was able to cross the bridge, and not just dropped by pure chance: because it was a green elf-stone.

I wonder if Frodo had heard about Luthien before, or did Strider's telling of that story leave such a deep impression that he called not only on Elbereth but on Luthien.

It is possible that he had heard of Luthien before from Bilbo, but I think it is more likely that the reason he used Luthien's name was because of Strider's story. Bilbo may have heard of her from the Elves and maybe even knew a good bit of her story, but he did not have the resources in the Shire that he did at Rivendell. Strider probably knew more and was perhaps better able to tell the story because he was raised in Rivendell, and it was his family history as well. Another interesting thing I wonder about is why he used Luthien's name at the ford but not when he was attacked on Weathertop. If it had been because of Aragorn's story, I would think that it would have been more fresh in his mind. On the other hand, maybe it hadn't quite sunken in yet, and being that Frodo was riding Bill and not walking, maybe he had more time to think about it, and it seemed fitting at the ford.