Ok, Im trying to complete puzzle 15 to get the last door key. Do you need to have all three levels of nature to complete that one because everytime I try to put my chief or master builder on the pearl it says they don't have enough skill to complete this object. Maybe I'm just doing something wrong. Can anyone help?

If I remember, you need mastery of a different skill, here. Since it's not marked spoiler I won't say more here, but a detailed solution is available in the "Guides" forum, if you want to check there.