One of the mysteries of the English language finally explained.
Crystalline alum of high purity, especially that prepared from Italian alunite.
Mid 17th century. From rock + alum, after French alun de roche (1374 in Middle French; compare Anglo-Norman alome de roche, alum Roche, and Middle French (Tournai) alun en roque) or Italian allume di rocca. Compare post-classical Latin alumen de roca, alumen de rocco, Spanish alumbre de roque, and also Spanish piedra alumbre, Portuguese pedra-ume, in same sense, which show substitution of the respective Spanish and Portuguese reflexes of classical Latin petra rock, stone.
rock alum/ˈrɒk ˌaləm/
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