According to Greek mythology, it was prophesied that one of Kronos' children would dethrone him, and so he swallowed each one, including Hestia, Demeter, Hera, Hades and Poseidon. When Zeus was born to Rhea, she tricked Kronos by giving him a stone wrapped in swaddling clothes and then hiding the infant on Crete. The child was nourished by the goat Amalthea and protected by the Curetes, semi-divine beings who inhabited Crete, who danced and clashed their shields to drown out the cries of the infant.
The subject was never very common in Greek and Roman art. From the same period as our gem there are two so-called Campana reliefs, one in the Museo Nazionale in Rome and one in the Glyptothek in Copenhagen, nos. 285 & 286 in Canciani, "Zeus/Iuppiter" in LIMC, p. 445. The subject is also known from coins and gems, but generally abbreviated to Amalthea suckling Zeus, such as the agate cameo in the Bibliothèque Nationale, Paris, no. 298 in Canciani, op. cit. Our seal is perhaps the fullest representation of the myth known from a gem.