Adamant
Adamant is any especially hard substance, whether diamond, other gem or metal. Both adamant and diamond derive from the Greek word adamas, meaning untameable.Since diamond is now used exclusively for the hardest gem, adamant has mostly poetic or figurative use. As an increasingly archaic word, it -- and its adjectival form adamantine -- is especially often seen in fantasy fiction. For instance, in J. R. R. Tolkien's The Lord of the Rings, Galadriel's ring Nenya is also named the "Ring of Adamant". Adamantium is another form found in science fiction and fantasy.
In the book Gulliver's Travels by Jonathan Swift, the bottom of the flying isle of Laputa is made of adamant. The gigantic loadstone in the Astronomers' Cave that enables the island to move is also supported by adamant.
Other uses of the word