Master Ore is needed to upgrade the Master Sword. Take two pieces of Ore to the Blacksmith in Hyrule to get the first upgrade, then take another two pieces of Ore to the Blacksmith in Lorule to get the strongest Master Sword.
Most of the pieces of Master Ore can be found in Lorule’s dungeons.
Requirements: Bombs, Merge
Instructions: Go to the top floor of the dungeon and enter the room with the boarded-up windows. Make sure that the platform at the back of the room is lowered so that you can stand on it. Go down and place a Bomb next to the switch at the front of the room. Get onto the platform at the back of the room and hit the switch next to it to reach the ledge above. Merge into the wall on top of the ledge before the Bomb explodes. When the Bomb triggers the switch, a section of the wall rotates. The small room on the other side of the wall contains the Master Ore.
Instructions: The Master Ore is in the large room on floor B3. Before you leave the room, enlist Thief Girl’s help to press the pair of buttons on the left. This reveals a shortcut through the room, a whole lot of Red Rupees, and a Treasure Chest containing the Master Ore.
Instructions: Near the end of the dungeon, look out for a big Treasure Chest on top of a wall. Go outside and find the hole that allows you to drop down onto the ledge nearby. From there you can easily reach the Treasure Chest by merging into the wall.
Requirements: Titan Mitt, Merge
Instructions: One piece of Master Ore is not found in a dungeon. Return to the Sanctuary in Hyrule anytime after you have retrieved the Titan Mitt from the Desert Palace. Inside the building is a Fissure. This is the only way to access the area of Lorule where the final piece of Master Ore is hidden.
On the other side of the Fissure is a cave. Go outside toward the graveyard and move the big rock in the top right hand corner to reveal stairs going underground.
The area underneath the graveyard is almost identical to the passageway that you explored at the start of the game. You can walk straight into the second room without solving any puzzles. The door to the third room is locked. To find the Small Key, follow the walkway until you reach the Treasure Chest, then merge into the wall and go right. Defeat the three Eyegores in the third room to move on to the fourth room. Pull the lever on the right to open the exit.
You will arrive in another part of the cave where you started. Merge into the wall on the right to reach the Master Ore.