How do you become a AAA in fossil fighters

Go to Rivet Ravine and go a little North west until you see a guy wearing a hat with bandages. Talk to him and you should start a little side quest. All you have to do is dig up 13 or 14 coloured fossil rocks and show them to him. Don't worry though, he won't take them. Then you get AAA grade and be able to dig up dark fossil rocks. Hope this helps.