Biome Key Molds

Biome Key Molds are extremely rare Hardmode crafting materials used to craft Biome Keys, which open Biome Chests. During Hardmode, every enemy has a 0.04% (1 in 2500) chance of dropping a Biome Key Mold. The type of Biome Key Mold that is dropped depends on the biome you are currently in (Jungle, Corruption, Crimson, Hallow or Snow). Any mold can drop in Ocean biomes.

Tips

 * Molds are an extremely rare drop, and can take hours to farm. Note that there is a limit to the number of items that can remain dropped at once, so leaving a farm running for hours unattended can result in molds being dropped and then disappearing due to the limit being reached.
 * Enemy spawn rates are higher during a Blood Moon, even underground, which makes a good time to farm for Molds. The Water Candle and Battle Potion also increase spawn rates.
 * Terraria's server software (or turning off "Autopause" on single player) can help farm for rare drops like Biome Molds by allowing the game to run in the background.