25 Combinations in the Nature category:

1. Acid Rain

2. Clouds

3. Coal

4. Cold

5. Coral Reefs

6. Fog

7. Fulgurite

8. Gas Field

9. Humus

10. Hydrothermal Vein

11. Lightning

12. Oil Field

13. Peat

14. Rain

15. Sedimentary Rock

16. Snow

17. Space

18. Star

19. Stromatolites

20. Sun

21. Swamp

22. Thunderstorm

23. Tropics

24. Volcano

25. Wind