Skylanders Spyro's Adventure soul gem locations guide

Gem #27: Arkeyan Armory

Unlocks: Voodood’s “Impervious Tripwire” Ability

As you near the end of the level, you’ll reach The Crucible. While descending the winding slope on your way to get the security key card, you’ll spot the Soul Gem tucked away in a corner

Continue on until you reach the security key card, and cut north to grab the Soul Gem at the end of the corridor.