A research group led by the Nagoya University Museum and Graduate School of Environmental Studies in Japan has clarified differences in the physical characteristics of rocks used by early humans during the Paleolithic. They found that humans selected rock for a variety of reasons and not just because of how easy it was to break off. This suggests that early humans had the technical skill to discern the best rock for the tool.
The researchers have published the results in the Journal of Paleolithic Archaeology.
Well of course they knew which rocks would make the best tools, having been doing it for millennia.