Some of the early stones used--heck some of the more recent ones as well--are pretty nondescript. A big slab of stone might have been used as a footrest, for all we know, right?
Archaeological evidence for the use of these stones as parts of grinding pairs is typically from two sources: usewear and plant residue analysis.
Usewear is macro- and microscopic damage, pitting, striations, smoothing and the like on the working surfaces of possible grinders. A slew of replication studies concerning usewear and how to recognize it were conducted in the 1970s and 1980s, and we think we have a pretty good handle on the process.
More recently, say within the past twenty years or so, there have been a wealth of studies on plant residues. During grinding hundreds or thousands of years ago, microscopic pieces of the organic material were ground into the coarse surfaces of the grinding pairs, and fused there by frictive heat or other chemical or mechanical processes. Starch grains, pollen and opal phytoliths have been used to successfully identify which plants were processed on grinding pairs in several cases dated to at least 30,000 years ago.
Sources
A bibliography has been built for this project.
