An unfashioned rock off the ground. When someone picks up a rock in his hand and strikes it repeatedly against an anvil of some kind (another rock, typically), it acquires an identifiable pattern of pits and scratches that remain indefinitely. At some point in the fossil record, you start finding rocks that exhibit this wear pattern, but before that, nothing. Apparently these things are common; after all, the "fossilization" rate is near 100%.
The key word being "repeatedly," meaning the same favored rock gets used. That means some practical place to keep it.
But stone tools can be much cruder than that, if the rocks are used once and then discarded. And no consistent wear pattern would result.
