Theoretically, Yes you can metal dectect in a graveyard/cemetary. There is dirt and some metal objects are left around (flower holders, pins, coins, etc). But ethically and in good image of the hobby, I would not. I say let the dead rest in peace. I would not do it. I know it can be tempting, but passers by from the road would just see you and label all of us as "grave robbers".