You can use heart scales at the Move Re-learner in Pastoria City. He reteaches your Pokémon a move for every heart scale used.
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You use the explorer kit you got from the old man in eterna city to go underground and mine at the sparkling spots on the walls eventually you will dig up a heart scale
you use it for the move maniac who reteaches your Pokemon a moves for one heart scale
you have to use hm rock smash on rocks to find them.
After torterra is able to learn it, if you select no then you can use a heart scale and then teach it the move using the heart scale tutor
one way to get a heart scale in pokemon black is to go to Driftveil city. Use fly to get there and then go left from the pokemon center, there a girl will ask you to show her a pokemon that knows a certain move. Find a pokemon that knows the move and in return she'll give you a heart scale.