Erronea onyx, common name the onyx cowry, is a species of sea snail, a cowry, a marine gastropod mollusk in the family Cypraeidae, the cowries.
The shells of these quite common cowries reach on average 32–38 millimetres (1.3–1.5 in) of length, with a minimum size of 24 millimetres (0.94 in) and a maximum size of 57 millimetres (2.2 in). The dorsum surface of Erronea onyx is smooth, shiny and generally golden brown, with alternating longitudinal fuzzy bands of translucent bluish and reddish colors (hence the Latin name onyx). The base and the margins are dark brown or black, sometimes the teeth are orange. In the living cowries mantle is dark brown, quite thin, with bluish papillae.