Marble Key
From Drunkapedia
A key that is carved from a Twisted Dark Key dropped by many monsters that spawn within the Valley of Death and carved using Lockpick skill with an Intricately Craved Tool. It is used to unlock a Reinforced Oaken Chest which is full of Sturdy Iron type loot and randomly spawns in the Valley of Death. In all the quality of the loot is not worth the hassle of using these keys and they have no other use or value to an NPC.