Diamond Egg
The Diamond Egg is the egg carried by Princess Shizuka. The Egg contains a powerful alien which Shizuka is entrusted with nurturing until it is ready to be born. Once the creature hatches, Shizuka mothers a new egg in its place. This has been Shizuka's role since she was abducted from Ridgemont High, hence her moniker "The Mother of All."
The Diamond Egg's shell is said to be the strongest material in The World, but weighs very little, making it very valuable. Due to such, it is requested by Skinny Ray to create armored plating for his Flying Machine. During the God UFO quest, Corobo visits Shizuka in the dead of night, only for her to refuse giving up the Diamond Egg until it has been born. After three nights, the Egg hatches, and Shizuka gives the shells to Corobo.
If Corobo talks to the Eggan at night, he will talk of a Golden Egg. This appears to be a remnant of beta content, in which a golden egg was a gift from Omelet to Shizuka.