[Voralphion]

I think it was the oath not the stone that kept the oathbreakers in limbo. In ME oaths are very powerful things and because they did not fulfil the oath, the spirits of the oathbreakers did not find rest and hung about the stone because that is where the oath was made.
With regards to the stone, I too thought it was an odd thing to bring on a boat if you are fleeing for your life but perhaps it was brought to use for building, as the Tower of Orthanc is said to be made of smooth black stones and I always thought that the stone of Erech was similar to the stones used there.