I guess every stone that has a meaning and is marked with a symbol could be a rune stone. I suppose they also exist in every culture in some way. You are right, the type of rune stones you have is often used for these kinds of things; and again I would argue that it would be comparable to e.g. the I Ching used in China etc. People like to know about the future it seems :)

I think "blaming" aliens for all of the past artifacts is definitely not useful as it just puts the whole past in one pot so to speak. Although I am definitely open to the idea for some of the bigger constructions/mysteries...