What is Adamant?

Adamant is a mythical material legendary for its toughness, but what actually is it, and why does it sound like diamond?

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Further reading:
en.wikipedia.org/wiki/Adamant
www.etymonline.com/word/adamant
www.etymonline.com/word/diamond
www.merriam-webster.com/word-of-the-day/adamantine…