Core of Earth

The Core of Earth is an elemental part of Azeroth and is useful in crafting powerful items.

Source
This item drops from Earth Elementals and Infernals.

Core of Earth as a quest item
Core of Earth is needed for the following quests:
 * (Alliance)
 * (Horde)

Core of Earth as an ingredient
Core of Earth is used in the following recipes:

Blacksmithing

 * Earthforged Leggings
 * Light Earthforged Blade
 * Runed Mithril Hammer
 * The Shatterer

Engineering

 * Lil' Smoky
 * Ultrasafe Transporter: Gadgetzan

Jewelcrafting

 * Figurine - Truesilver Crab

Leatherworking

 * Volcanic Leggings

Enchanting

 * Enchant Cloak - Greater Resistance