Soul Gems are items available in Skylanders: Spyro's Adventure, Skylanders: Giants, and Skylanders: Swap Force that unlock upgrades for individual Skylanders and can be found in all playable levels of both games.
After the gem has been found in a level it can be given to Persephone in exchange for the skill available.
Something to note is that only Adventure Pack Soul Gems can be found in the levels they come with.
 Soul Gem Locations
 Spyro's Adventure
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