Well, for one, the rock is natural, likely only acted upon by natural forces. It is, likely, homogeneous in material, though that's not guaranteed. It performs no particular preplanned function, though it may, in actuality, function as a paperweight. There is no way to, repeatedly encode new information into it's structure, and it may last, as it is, for thousands of years.
The watch, OTOH, is made of several raw materials. The materials have been mined and purified and shaped and fitted by human intelligence, to operate together to perform the preplanned function of tracking the passing of time. The watch has a mechanism whereby new information and energy may be encoded into it as often as necessary for it to continue to function. The watch may also, be used one day as a paperweight, and it will doubtfully continue to last, as it is, for much longer than a few years.