As you cross Guardian Gangway, you'll approach some swirling pools. Drop into the easternmost pool to find a Soul Gem in a secret area.
Take a left before the first set of mines on Hazardous Highway. Follow the short path to find this.
Before exiting the Underground Lake, destroy the crates near the exit to reveal a blue bounce pad. Use it to reach the rotating platforms above; cross them to nab another Soul Gem.
This one's really easy to get. When you enter Gift Boat Row, take a left and hop across the spinning platforms to reach it.
After riding the geyser rock in Paleo Pass, you'll be near some small platforms with more rocks to push. Push the closer one off of the ledge, then drop down and give it another push. It can now be used to reach the Soul Gem on the high ledge you passed earlier.
Before riding the rails at the end of The Frozen Curtain, hop up the platforms to your left. There's a small gap in the fencing near the laser where you can drop behind the top platform. Turn the wheel hidden back there to activate platforms which will allow you to reach the Soul Gem.
When you free the final pair of snowmen in Diamond Docks, head out onto the ledge where the eastern snowman was frozen. Hop across the platforms here to reach another Soul Gem.