Primal Fire

Primal Fire is a molten chunk of pure fire goodness. It is used in a wide range of high-end recipes. In addition it is also considered the most in demand, and thus, profitable primal for farming.

Source
Primal Fire is made by combining 10, which can be found off fire elementals or in Fel Iron deposits. A guide to finding these can be found here.

Primal fire can also be created through Alchemy with the Transmute: Primal Air to Fire and Transmute: Primal Mana to Fire recipes.

Primal Fire as an ingredient
Primal Fire is used in the following recipes:

Enchanting

 * Enchant Bracer - Spellpower
 * Enchant Cloak - Major Resistance
 * Enchant Shield - Resistance
 * Enchant Weapon - Sunfire

Mining

 * Fire Sunder - creates 10 Motes of Fire