The answer to this can be found in Chapter 15 of Guns, Germs, and Steel. To be precise, though, Diamond does not explicitly tell us where the very oldest stone tools with ground edges have been some of these tools have been found. The answer is that they have been found in Australia. As Diamond says on page 297 in the paperback edition,
Native Australians developed some of the earliest known tools with ground edges…
This is important to Diamond because he talks about how people tend to believe that the natives of Australia were inherently backward. Diamond rejects such arguments about people’s cultures. He believes that geography is what is important, not culture. Therefore, it is important to him that the Australians were able to create innovative technologies. It shows that they were not incapable of creating technology. He can then go on to explain why the Australians ended up at a relatively low level of technology by the time the Europeans got to Australia.