Single octagonal cufflink face in white metal, with a narrow blue enamel band running around the inner edge, a black enamel or oxidized ring inside that, and a central raised bezel with a fluted/beaded edge holding a pale green-white cabochon (possibly chrysoprase, jade, or an imitation/glass stone). Only the decorative face is visible in this photo; the reverse fitting (toggle, chain, or post) is not shown.
A dress or shirt cufflink typical of early-to-mid 20th-century design, combining enamel banding with a central hardstone or glass cabochon.