In search of the gem

Go to Mt. Ember, and go right after going up the stairs. You'll find two Rocket members talking to each other about the secret passwords ("Goldeen need log"), blocking an entrance. Go up to them and battle both of them. They'll leave after you defeat them. Go inside the cave and follow the path. You'll go through a boulder puzzle, and then reach a fork in the road. Take the south road; the other road takes you out. The next room is full of Braille messages. Go on to the next room.

Pick up the Ruby in the center of the room. Leave Mt. Ember, and go back to Celio. Hand him the Ruby, and he'll want you to go onto another mission. You'll hand him your Tri-Pass, and he'll give you the Rainbow Pass, used for exploring the rest of the islands. Now take the Ferry to Four Island.