A corruption of adamant. So called because the diamond, which cuts other substances, can be cut or polished with no substance but itself. (Greek, a damao, what cannot be subdued. Latin, adamas, gen. adamant-is; French, diamant.)
“As for Warrington, that rough diamond had not had the polish of a dancing-master, and he did not know how to waltz.” —Thackeray.
Diamond cut diamond. Cunning out-witting cunning; a hard bargain over-reached. A diamond is so hard that it can only be ground by diamond dust, or by rubbing one against another.
(Newton's favourite little dog). One winter's morning, while attending early service in Trinity College, Newton inadvertently left Diamond shut up in his room. On returning from chapel he found that the little fellow had upset a candle on his desk, by which several papers containing minutes of many years' experiments, were destroyed. On perceiving this irreparable loss, he exclaimed, “Oh, Diamond, Diamond, thou little knowest the mischief thou hast done!” (Diffusion of Useful Knowledge: Life of Newton, p. 25, col. 2.)
Huygens, 1694, referring to this accident says: “Newtonum incidisse in phrenitin abhinc anno ac sex mensibus. An ex nimia studii assiduitate, an dolore infortunii, quod in incendio laboratorium chemicum et scripta quædam amiserat.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894