Harmonium is an item that is used for synthesis in Atelier Sophie 2: The Alchemist of the Mysterious Dream. It is needed to make high level weapons in the game. Its recipe is a bit complicated to learn. However, this guide will help you unlock it.
To learn the recipe for Harmonium, you need to:
Atelis Ore is easy to obtain. It’s a LV30 material that can be gathered at Snowy Corridor, Near-Forest, Almare Grasslands, Thunderbolt Palace, Eternal Sand Dunes, and Erk’s Ice Caves. It can also be dropped by the Stone Puni (white/grey color Puni).
Priarco, on the other hand, requires that you learn its recipe and craft it.
Priarco (LV59)
Diamond Golem is a monster that can be found at Eternal Sand Dunes and Quartz Lapis. I personally went to the Oblivion Wasteland in Eternal Sand Dunes (the second area of Eternal Sand Dunes). In this area, Diamond Golem can be found northwest of the map.
Return to your Atelier and craft Priarco (and complete the other tasks) to unlock the Harmonium recipe.
