Adamant

Adamant – Adamant was a stone that once believed to be impenetrable, giving rise to the saying: "As hard as adamant." (Daniels, Encyclopaedia of Superstitions)

It may be an archaic word for diamond.  Because adamant was believed immune to fire, artisans would soak the mineral in goat's blood to soften it.

In medieval times, it was believed the strength of the gem could be transferred to it's owner, making them strong against their enemies. It was also thought to repel vanity, venom, and lunacy. (Duffin, Fifteenth century magico-medicinal minerals)

Adamant pops up often in Greek mythology.

Kronos was given a sickle made of adamant by his mother Gaia, which he used to castrate his father Uranus. (Hesiod, Theogony)

The gates of Tartarus are framed with pillars of solid adamant. (Virgil, Aeneid)

Cupid/Eros held Psyche, daughter of Sol, captive with chains of Adamant (Martianus Capella, De nuptiis Philologiae et Mercurii).

Hephaestus used adamantine bonds to bind Prometheus to his place of punishment. (Aeschylus, Prometheus Bound)

And the hero Perseus was given a special adamant sword called a harpe (see image below)  so that he could slay the gorgon Medusa. (Pseudo-Apollodorus, Bibliotheca; Pseudo-Hyginus, Astronomica)