A mordant is a metal piece, sometimes jeweled, affixed to a belt or bracelet on the opposite end from the buckle. The purpose of a mordant is to facilitate the sliding of the end through the buckle and under the slide. In jewelry a mordant usually serves as a decorative way to finish the non-buckle end of a strap bracelet.

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Ruby and Diamond Ludo Bracelet with Coordinating Buckle and Mordant.
Photo Courtesy of Lang Antiques.