E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.)
Diamond (3 syl.). Son of Agapë, a fairy. He was very strong, and fought either on foot or horse with a battle-axe. He was slain in single combat by Cambalo. (See TRIAMOND.) (Spenser: Faërie Queene, book iv.)
A diamond of the first water. A man of the highest merit. The colour or lustre of a pearl or diamond is called its water. One of the first water is one of the best colour and most brilliant lustre. We say also, A man of the first water.
A rough diamond. An uncultivated genius; a person of excellent parts, but without society manners.
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 outwitting cunning; a hard bargain overreached. A diamond is so hard that it can only be ground by diamond dust, or by rubbing one against another.