Identifying niello and inlay
Black metal decoration.
Niello is a black metal-sulfide paste inlaid into engraved lines on silver or gold and fired to a deep, even, slightly glossy black filling. It was prized from Roman silver through Anglo-Saxon and medieval work.
Distinguish it from tarnish or plain dirt in the lines: tarnish wipes or polishes away, niello does not, and under a loupe niello shows as a solid fill standing flush with the surface, often with tiny losses where it has fallen out of the cut.
Damascening — hammering precious-metal wire into cuts in iron or steel — is the related inlay tradition, and survives as gold or silver lines in a corroded iron surface.
Never polish an object with suspected niello or inlay; polishing lifts the very thing worth keeping. Record where the black fill or wire survives and where only the empty engraving remains.