Inlay material on some guitars

NACRE

This iridescent shell layer is often called "mother-of-pearl" in decorative work.

This clue refers to the shiny, iridescent material often used in guitar inlays, commonly known as mother-of-pearl. "Nacre" is the technical term for the same substance, which comes from the inner layer of mollusk shells and adds decorative flair to fretboards or headstocks. The answer plays it straight—no wordplay—just a direct reference to the material’s formal name.