Originally Posted by Saddlebag
The rocks will help toughen her hooves.
...Assuming they're healthy enough in the first place. If not, forcing her to remain uncomfortable on her feet may just make matters worse, as she'll exercise less & likely not use her feet properly either.
Assuming she is sore, try getting her to stand the other foot on some thick foam or polystyrene. Do you know what's going on with her feet? They shouldn't be very sensitive to rocks/hard ground when she's just standing around, unless there's an underlying cause, such as laminitis for eg.