Stone of Change/Soul Gems

Stone of Change Crafting

 * Soul Gem: Essence of Earth
 * CR7 Earth Elemental (The Depths of Discord on Hard, The Depths of Despair on Elite, The Iron Mines: Justice for Grust (part 2 of Grey Moon Waning), Taming the Flames on Hard has 20+ Earth Elemental)

Note: This a regular soul gem with no suffix or prefix, thus uses the 20HD version of the spell.