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There are two ways to obtain this Materia.
1. Defeat Emerald Weapon and Ruby Weapon to obtain a Gold Chocobo. Time Consuming
2. Breed a Gold Chocobo. In my opinion EXTREMELY Time Consuming.
I highly recommend to use the "cheats" in the game and defeat the weapons. You have to do this anyway for achievements.
The Secret Achievement achievement in Final Fantasy VII worth 204 pointsContinue playing to unlock this secret achievement
The Secret Achievement achievement in Final Fantasy VII worth 210 pointsContinue playing to unlock this secret achievement
First defeat Emerald Weapon to obtain the Master Summon Materia (all summons in one materia even those you don't have)
I used a team with Barret, Cid and Cloud all with level 4 Limit brakes.
Barrets 4th limit can you get in Old Corel after the train sequence later in the game.
Cids 4th limit can you get in the sunken Gelnika ship also later in the game.
Clouds 4th limit can be super time consuming. You have to battle in the battle square inside the Gold Saucer "on disc 3" and get 32.000 battle points to buy his limit. And here is the point what makes this also time consuming. You need 10GP (Gold Saucer points) to battle. For each battle you can get around 3000-10.000 Battle Points. And if fail a battle for this fight you get 0. There are many ways to obtain GP. The Easiest is to "buy" them. Sometimes there is a guy on this spot at the Gold Saucer.
If not just leave and come back again. Here you can buy up to 100GP vor 10.000Gil. I highly recommend to rack up 1000GP for 100 battles.
After you have enough GP you can go battle. The battle mechanics are more ore minor complex but what we do we use the "cheats". I used any weapon and any armor (no big differences) but what i recommend is a Ribbon as an accessory.
Up to this point you can have up to two ribbons.
One in the Temple of the Ancients: You reach a room with a giant Clock. Leave the Clock at 5 there is the ribbon.
The second one is in Gaea's Cliff: Here is a video for this ribbon
(Video Credit goes to LetsPlayGGs)
In the battle square you have to fight up to 8 enemies. For each consecutive win you get more BP (battle points). The more the higher ( 
). From round 3000 up to 10.000. That means 4 to 11 battles if you never lose.
But here is what it makes time consuming. After every battle you have a slot mechanik with a debuff. Broken Weapon, Broken Armor, Broken Accessory, Broken Materia of any or all colors, Lvl Down, HP/MP Down and so on. If your accessory is broken its often, without better gear and Materia, impossible to win. Just play again and again up to you 32.000 Battle points. Buy the Omnislash and you are done.
After all this preparations, buy some HP+ Materia,
but a maximum of 6 Materia in total for each character, in Cosmo Canion, to get 9999HP on each character. Also reach level 75+ in the north crater. See this solution.
The Liege of Leveling achievement in Final Fantasy VII worth 108 pointsReach Lv. 99 with one character
And then follow this video.
After the battle you obtain the Earth Harp item. This can be traded in Kalm to one Master Materia of all three types, Command, Magic and Summon.
Now take Cloud or what ever you are most comfortable with. And defeat Rubi Weapon in Gold Saucer dessert. Have cheats enabled HP to 9999 and use as Materia a Ultima or Comet Materia in a Combo Slot with Element Materia and the Master Summon Materia.
At the beginning Rubi "removes" 2 characters from the battle. Reload the save up to the point your character with the Master Materia stays in battle.
You can also make sure the Character with the Master Materia doesn't get removed by starting the Battle with the other 2 Characters dead. Make sure you don't have Cheats on when you start the Battle or else they will be revived, once in Battle you can reactivate the cheat.
Just spam Knights of the Round on Rubi till he is dead. Some times he spawns his claws. Kill them with a Knights of the Round as fast as possible!.
If you like you can buy the W-Summon Materia for 64.000BP in the Gold Saucer. This grants you the W-Summon (Double Summon) ability. It allows to summon another summon direkt after your first. You can then just double Summon Knights of the Round. If you summon Knights of the Round at first and as your second summon Hades there comes a handy effect that stops Rubi to cast his Ultima counter attack.
There are many many other good ways and strategies to finish this Super Boss. But nearly all of them are a lot more time consuming than the strategy in this guide.
After the fight you obtain the Desert Rose item. This can finally be changed to a Gold Chocobo also in Kalm. If you can't store your Chocobo (it will stay in the room) fly to the Chocobo Farm and rent some barns. Come back and take the Chocobo. The Chocobo Farm is East-Southeast of Kalm.
After all this stuff. Just walk to the "secret" island and take the Knights of the Round materia. Done!
here is a video of the Walk to the Island. (you don't need the highwind you can walk direct over the ocean to the island.
Video Credit goes to bboytips
