In this case I think cobblestone is correct? They tested natural stones (Merriam Webster: cobblestone - "a naturally rounded stone larger than a pebble and smaller than a boulder") vs the hand made stones for this particular task.
"cobble" is more frequently a verb in my experience, to "cobble something together". The dictionary does say cobblestone as a noun = cobble as a noun though. So although they seem to be equivalent I think the original objection is that in some dialects / regions it probably just flows better in a sentence? Probably another subtle british vs american english thing. (I'm american) -- edit My brit friend says it's a stone until it's in a road, then it's a cobblestone.
Cobble is what a geologist would call a medium-sized water-rounded piece of rock if it was free or embedded naturally in a matrix (like glacial till or sediment). They’d only call it a cobblestone in the context of like, paving a street or driveway or something.
However, I don’t think that makes it wrong for people / other disciplines to call it a cobblestone.
"cobble" is more frequently a verb in my experience, to "cobble something together". The dictionary does say cobblestone as a noun = cobble as a noun though. So although they seem to be equivalent I think the original objection is that in some dialects / regions it probably just flows better in a sentence? Probably another subtle british vs american english thing. (I'm american) -- edit My brit friend says it's a stone until it's in a road, then it's a cobblestone.