Skylanders Spyro's Adventure soul gem locations guide

Gem #17: Battlefield

Unlocks: Drill Sergeant’s “Arkeyan Armor” Ability

Shortly into the level, you’ll come across this locked gate. You might notice a Soul Gem peeking out from behind the corner that stone wall. Note the weapon rack and rock wall on the right.

Take out the rack and wall with any attack, and grab that Soul Gem.