According to Willet and Thompson, didon, a "luminosity, or shining smoothness of surface" (Willett, 213), such as appears in the examples above, is a favored quality in Yoruba -- and, one assumes, much other -- African sculpture in various media.
[Left: Ibo Queen Victoria; collection Pitt-Rivers Museum, Oxford University.]
[Center: Reliquary Guardian, Fang (Gabon), collection Carlo Monzino; illustrated Vogel, 23.]
[Right: Geh Porro society; illustrated in Richard Huber, Treasury of Fantastic and Mythological Creatures, NY: Dover, 1981, pl 121 (no pages).]