in the empire of ice, but i didn't collect it. it's hidden somewhere!
get empire of ice and start looking
You find it on Cadavourus Crypt
If you buy Pirate Seas, the soul gem is something with a cannon. I'm not sure where it is, but it is definitely in Pirate Seas. If you go by the cannon on land go right up the stairs and you will find the soul gem.
dark light krypt
in the level empire of ice but you have to buy it
you find the soul gem in darkwater cove but i dont know the exact place in the level
You can not get all the soul gems in the dark cove on skylanders.
as well as Terrafin, sunburn and ghost roaster you find it in his adventure packs level
in Empire of ice where the first maze is
GET soul gem and unlock the soul gems upgrade
Dragons peak
I'm pretty sure you find it in dragon's peak which you have to buy to unlock and it comes with sunburn.