Terraria (Wiki): Obsidian Skin Potion

Obsidian Skin Potion can be found in Azure Crate (4.14%), Boreal Crate (4.17%), Bramble Crate (4.17%), Corrupt Crate (4.17%), Crimson Crate (4.17%), Defiled Crate (4.17%), Divine Crate (4.17%), Dungeon Crate (4.17%), Frozen Crate (4.17%), Golden Crate (6.67%), Hollowed Crate (4.17%), Hellstone Crate (4.17%), Hematic Crate (4.17%), Iron Crate, (3.13%), Jungle Crate (4.17%), Mirage Crate (4.17%), Mythril Crate (3.13%), Oasis Crate (4.17%), Obsidian Crate (4.17%), Ocean Crate (4.17%), Pearlwood Crate (1.43%), Seaside Crate (4.17%), Shadow Crate (9.38%), Sky Crate (4.17%), Stockade Crate (4.17%), Titanium Crate (6.67%) and Wooden Crate (1.43%)