Hint: Gems of Hoenn - Guide for Pokemon: LeafGreen
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The Ruby After obtaining the National Pokedex in Pallet Town, go talk to Celeio on One Island. He'll ask you to find him a gem on One Island, otherwise known as the Ruby. To get the Ruby, Surf on Kindle Road, and keep going straight. Enter Mt.Ember and walk a little way to the right side of the mountain. There will be to Team Rocket members talking about the Ruby and the passwords to enter the Warehouse on Five Island. One of the Rockets will say the first password, then the other member will have seen you and end the conversation. Battle the two Team Rocket members, and make sure you don't lose to them. After defeating them, enter the cave. Follow the trail on left side of te cave. After the third set of ladders, you'll come across a fork (not the one you use to eat, mind you). Anyways, take the left way to where there is a a boulder. Have a Pokemon that knows how to use Strength to move the boulder. Then you'll come across a room with Braille writing, which I really can't understand. Ignore the Braille and go on. In the next room, you'll see obtain the Ruby. The Sapphire After giving the Ruby to Celio, he'll ask you to do him another favor. He'll ask you to find the Sapphire for him. He'll give you new pages for your Town Map, and then he'll also trade your Tri-Pass for his Rainbow Pass, giving you access to the Sevii Islands. Take the boat first directly to Six Island, and go to the place on your map known as the Dotted Hole. Once there, stand in front of the door and use Cut. Enter the cave and fall down the hole in the ground. You'll end up in a cave with four holes. Fall down the holes in the following order: Up, left, right, down. Press A on the Sappire, and the Scientist Gideon will appear. He'll steal the Sapphire, and give you the second password to the Warehouse. From here, take the boat to Five Island, and then press A on the Warehouse door in the Five Island Meadow. You'll automatically say "Goldeen need log", and "Yes, nah, Chansey." The door will then open. Find your way into the upper-back side of the Warehouse, and battle the Rocket Administrator. Defeat him and then battle Scientist Gideon. After winning, he'll give you the Sapphire in rage, and ask you to leave him be. Take the last gem to One Islang and then give it to Celio. In a few seconds, the right side of the computer will turn Blue, while the left side of the computer will turn red. You can now trade Pokemon with Ruby/Sapphire/Emerald, and Gold/Silver/Crystal Versions. (Note: Once you've traded a Pokemon from Leaf Green Version, you'll automatically get the National Pokedex.)