Skylanders Spyro's Adventure soul gem locations guide

Gem #26: Quicksilver Vault

Unlocks: Wrecking Ball’s “Enemy Slurp” Ability

About halfway through the level, you’ll reach this locked gate.

Take a left to find a Soul Gem guarded by a rather tough enemy, the Arkeyan Ultron. You could just run behind him and grab the collectible, but you’ll have to defeat him to progress in the level anyway.