NIELLO is validvalid

a black metallic substance [n NIELLI, NIELLOS] : NIELLIST [n] / to decorate with a black compound of sulfur [v NIELLOED, NIELLOING, NIELLOS]

24,465 of 196,601 in NWL2023·also valid in CSW24

(Italian) a black metallic substance [n NIELLOS or NIELLI] / to decorate with a black metallic substance [v -ED, -ING, -S]