I know natural stones like Escher's and Coticule's were around back then, but when did synthetic hones come onto the scene? Were they around back then too?
Coticules were discovered in the 1950's...I have my wife's grandfather's...father's honing stone along with its "slurry paste"..seemed like pressing raw materials together..then baking over very high heat..thus forming a hard but smooth stone like surface for rehoning/touchups
I'm pretty sure cavemen (or damn near that far back) rubbed mud and sand onto flat surfaces to sharpen tools on. That would be the first synthetic whetstone I'm aware of.
That wouldn't make it the first synthetic but the first paste.