You can earn diamonds in several ways:
  1. By collecting Free diamonds once a day in the Shop;

    Free diamonds
     
  2. By buying them;
  3. By completing sawmill missions;

    Sawmill missions
     
  4. By completing lumber pass missions;

    Lumber pass missions

  5. By participating in limited-time events;

    Event milestones

  6. By accumulating diamonds in the piggy bank and breaking it.

    Piggy Bank