Magnesium. According to GLaDOS, at least.
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You have to build a structure with obsidian and set the inside on fire with flint and steel. The structure cannot be made in your crafting table. You put obsidian in an upright 4x4 design, with the middle empty. Or if you have the portal gun mod first craft the portal gun then right click to fire portal and press g to switch between portals
thinkingwithportals.com
It does not exist. It is just a lie to get people to play the game. The owner should get reported for it, but people are too busy searching for this gun.
It's a mod. I do have it I would recommend it. But anyway it's a mod sorry pal.
You have to use a crafting table in minecraft you dont need to download it