Etymology: Middle English diamant , -aunt , < Old French diamant (= Provençal diaman , Catalan diamant , Italian diamanto , Old High German demant ), < late Latin diamas , diamant-em (medieval Greek διαμάντε ), an alteration of Latin adamas , -antem , or perhaps of its popular variant adimant-em (whence Provençal adiman , aziman , ayman , Old French aïmant ), apparently under the influence of the numerous technical words beginning with the prefix dia- prefix¹, Greek δια-. The differentiation of form in late Latin was probably connected with the double signification acquired by adamas of ‘diamond’ and ‘lodestone’ (see adamant n. and adj.); for, in all the languages, diamant with its cognates was at length restricted to the gem, as aimant was in French to the lodestone. In English the dyamaund and adamaund are distinguished from and opposed to each other c1400 in Maundevile, ed. 1839, xiv. 161, ed. Roxb. Soc. xvii. 80; but adamant long retained the double sense of late Latin adamas : thus Sherwood, 1623, has ‘An Adamant stone, (French) aimant , diamant , calamite , pierre marinière .’ See adamant n. The a of the middle syllable has tended to disappear since the 16th cent., as shown by the spelling di'mond, dimond. Sheridan and other early orthoepists recognize the dissyllabic pronunciation, but most recent authorities reckon three syllables. In Shakespeare the word is more frequently a trisyllable; but it is very generally dissyllabic in Pope, Thomson, Young, Cowper, Keats, and Tennyson.

Etymology: < diamond n.Previous versions of the OED give the stress as: ˈdiamond.