Steel Amulet: When you mine an iron ore, you get 1 steel block. Very useful. (Crafting: Combine 81 of the drops from Megastructure Ore. Megastructure Ore spawns rarely below Y = -4, and is bright green.)
Super Steel Amulet: Get 4 steel blocks per 1 iron mined. (Search "amulet" in crafting to view recipies.)
Skyscraper Amulet: Get 9 steel blocks per 1 iron mined.
Coal Amulet: Get 3 coal per 1 coal mined. (Search "amulet" in crafting to view recipies.)
Super Coal Amulet: Get 9 coal per 1 coal mined.
Your strongest amulet for a specific material determines your drops for it. Simply have the amulet in your inventory to get the effect.