adamantium

Momoは想像して遊ぶのが得意です。
Being a research vessel, of course, the Argo had been specially designed to tackle the Travelling Tornado. Her hull was entirely constructed of adamantium, a steel as tough and flexible as a sword blade, and had been cast in one piece by a special process that dispensed with rivets and welded seams.
"adamantium" はスティールの様に硬い金属であるのは文章からも分かりますが、聞いたことのない金属なので調べます。
・Wikipedia: Adamantium is a fictional metal alloy appearing in American comic books published by Marvel Comics, and is best known as the substance bonded to the character Wolverine's skeleton and claws. Adamantium was created by writer Roy Thomas and artists Barry Windsor-Smith and Syd Shores in Marvel Comics' Avengers #66 (July 1969), which presents the substance as part of the character Ultron's outer shell. In the stories where it appears, the defining quality of adamantium is its practical indestructibility.
漫画で作られた想像上の金属でした。小説Momoは1973年の発行ですからWikipediaに記述のある漫画で生まれた金属と判断して良さそうです。