The kingdom is mentioned in the Periplus of the Erythraean Sea as an important market place for ivory, which was exported throughout the ancient world. Aksum was at the time ruled by Zoskales, who also governed the port of Adulis. The Aksumite rulers facilitated trade by minting their own Aksumite currency. The state also established its hegemony over the declining Kingdom of Kush and regularly entered the politics of the kingdoms on the Arabian peninsula, eventually extending its rule over the region with the conquest of the Himyarite Kingdom.