Etherium Ingot
Don't let this name confuse you. It is physically solid. Very.
It can be smelted in either Furnace or a Blast Furnace out of Etherium Ore:
Smelting Etherium Ingot takes twice as much time as regular smelting recipes.
Etherium represents "better than diamond" tier of equipment, available in endgame stages of progression, after reaching the Outer Islands in The End. Tools and armor made of Etherium not only have high durability and effectiveness on their own, but also possess unique abilities that make them incomparably more powerful than anything vanilla Minecraft has to offer.
The ore, which is required to obtain the ingots, can only be found in End Cities. Collecting enough of Etherium Ore can be quite the challenge, especially if player is amining for full set of tools and armor. Classically, tools require less ingots than armor pieces - but it is slightly compensated by requirement of Ender Rods, which in order require Astral Dust to be crafted.
Tools and armor than can be crafted out of Etherium currently include: