Lava Core

Internal core of a very powerful, flame-based elemental. Used in numerous high-level craftable items often paired with Fiery Cores.

Source
These are rare drops from most trash mobs and guaranteed drop from some bosses in Molten Core.

Lava Core as an ingredient
Lava Core is used in the following recipes:

Blacksmithing

 * Black Amnesty
 * Blackfury
 * Blackguard
 * Ebon Hand
 * Fiery Chain Girdle
 * Fiery Chain Shoulders
 * Dark Iron Bracers
 * Dark Iron Boots
 * Dark Iron Destroyer
 * Dark Iron Gauntlets
 * Dark Iron Helm
 * Dark Iron Leggings
 * Nightfall (axe)
 * Sulfuron Hammer

Engineering

 * Core Marksman Rifle
 * Biznicks 247x128 Accurascope

Leatherworking

 * Black Dragonscale Boots
 * Chromatic Gauntlets
 * Corehound Boots
 * Lava Belt
 * Molten Belt
 * Molten Helm

Tailoring

 * Flarecore Leggings
 * Flarecore Mantle
 * Flarecore Robe

Lava Core as a quest item
Lava Core is needed for the following quests: