Cows are often associated with the plough and the harvest. In collaboration with the amphorae, the gem likely represents ‘plenty’ and is possibly votive, meant to encourage wealth and prosperity in the wearer. Cattle were a prolific source of meat in the ancient world and therefore seen as a great luxury.
Pliny the Elder records in his Natural History that the Magi believed that amethysts were thought to prevent drunkenness. The conjunction of iconography with this magical association with the material seems unlikely to be coincidental.