You save the current rock in an object.
Once this is mined another object will be placed and the old object doesn't exist anymore.
Last time I checked it's impossible to interact with something that doesn't exist
You'll have to update your rock list frequently to make sure you got the latest objects
(I tried to interact with my non existing girlfriend once before, unfortunately nothing happend... :'( Should have null checked her before ... -_- )
Khaleesi