[sharun]

Stone degrades faster than bone?

Stone tools have been found that are 2-3 million years old - https://en.wikipedia.org/wiki/Stone_tool

[spydum]

always wondered how exactly this process works.. you can date the stone itself, but how do you date the shaping of the stone?

[stevenwoo]

The assumption is the surrounding sediment can suffice for a good enough approximation. http://www.nytimes.com/2011/09/13/science/13qna.html