Obsidian Heart
From Drunkapedia
Obtained: Random Loot Trophy off of Obsidian Golems and Basalt Golems
Uses:
Carve it into 3 Obsidian Keys using lockpick skill with an Intricately Carved Tool. Obsidian Keys are used to unlock Large Mnemosynes.
or
Turn it into a Stone Collector for a small amount of xp and pyreals