Adamantium

Adamantium is perhaps the strongest substance known to the Imperium, and is invulnerable to attacks from most known weapons. It was the material used to construct the Imperial Palace's Eternity Gate, and is often used in conjunction with plasteel and ceramite to produce items that require incredible tensile strength, such as in the structure of Terminator Armour or the construction of Imperial Titans. Many items made in the past from adamantium cannot be reverse-engineered by the Imperium's Adepts because their adamantium shells are so strong that they cannot be disassembled.

Sources

• Warhammer 40,000 Rulebook (5th Edition)