is really the mineral corundum; but the word is indifferently used for rock crystal, diamond, or any hard substance, and also for the magnet or loadstone. It is often used by poet for no specific substance, but as hardness or firmness in the abstract. Thus, Virgil, in his Æneid vi. 552, speaks of “adamantine pillars” merely to express solid and strong ones; and Milton frequently uses the word in the same way. Thus, in Paradise Lost, ii. 436, he says the gates of hell were made of burning adamant:
This huge convex of fire Outrageous to devour, immures us round Ninefold, and gates of burning adamant Barred over us prohibit all egress.
Satan, he tells us, wore adamantine armour (Book vi. 110):
Satan, with vast and haughty strides advanced, Came towering, armed in adamant and gold.
And a little further on he tells us his shield was made of adamant (vi. 255):
He [Satan] hasted, and opposed the rocky orb Of ten-fold adamant, his ample shield A vast circumference.
Tasso (canto vii. 82) speaks of Scudo di lucidissimo diamante (a shield of clearest diamond).
Other poets make adamant to mean the magnet. Thus, in Troilus and Cresida, iii. 2:
As true as steel, as plantage to the moon, As sun to day, as turtle to her mate, As iron to adamant.
(“Plantage to the moon,” from the notion that plants grew best with the increasing moon.)
And Green says:
As true to thee as steel to adamant.
So, in the Arabian Nights, the “Third Calendar,” we read:
Adamant is a (negative) and damao (to conquer). Pliny tells us there are six unbreakable stones (xxxvii. 15), but the classical adamas (gen. adamant-is) is generally supposed to mean the diamond. Diamond and adamant are originally the same word.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Adamant from Infoplease: