How to clean a rock (sentimental reasons)?

My girlfriend and I always joke around about how even if we got each other rocks for each other's birthdays we would be happy with it. Well she is on a trip for the summer and me and a buddy were hanging around in this giant dustbowl behind my house and I found this piece of shale that is shapped exactly like a heart an I wanna give it to her. But I want it to be like super OCD clean. Any tips? It's shale so it's brittle and could break easily