Merriam-Webster Collegiate Dictionary - niello

 I. noun  (plural nielli or ~s)  Etymology: Italian, from Medieval Latin nigellum, from neuter of Latin nigellus blackish, diminutive of niger black  Date: 1816  1. any of several black enamel-like alloys usually of sulfur with silver, copper, and lead  2. the art or process of decorating metal with incised designs filled with ~  3. a piece of metal or an object decorated with ~  II. transitive verb  Date: 1866 to inlay or ornament with ~