Diamond Walls used to build a structure in Creative mode.

Diamond Walls are the second strongest walls in the game, made from 2 diamonds. Diamonds are costly, yet useful blocks for making shelters that can withstand attacks from dragons. Despite their great strength, players generally prefer to use them as decor instead to show off their wealth of their world.
Diamond Wall

A display of the Diamond Wall.

However, they can still be destroyed from attacks from the Undead Dragon and the RPG Weapon.

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