The Stone Poem is available on Home Island at level 13 - Complete the "Get the Onyx Arch" and "Fix the Onyx Bridge" quests to unlock it.
The Stone Poem can be upgraded 10 times throughout the game, with each upgrade requiring a different set of items. When fully constructed, it consists of 10 “poems” with each one representing a Gaia: Star, Sea, Forest, Earth, Desert, Jungle, Sun, Ice, Volcano, and Swamp. As you upgrade and add poems, the structure will grow like a totem pole.
For more info on upgrading the Stone Poem, check the guides we have posted to our official Facebook group and Discord channel.
For more information on exploring the islands and picking up the items you need to upgrade the Stone Poem, check out the Archipelago Islands section!