Most gems are in plain sight (or aren't hard to find if you explore the level). The easiest ones to miss are:
- The 2 red gems and an extra life on top of a mast early in the level. You need to tail-hang off a pipe to the left, spin, jump, and rocket boost up to it.
- When you unblock the conveyor belt and get to the top of its building, make sure to jump off the left for a bunch of gems, a heart, and an extra life.
- Late in the level (after some hang-gliding enemies), there's 1 red gem over a small wooden platform that has a small heart. It's just off the screen and easy to miss.
- There are two sets of hidden gems (one set is 1 red, 1 blue, and the other is 2 red, 1 blue) high above pipes late in the level. You need to tail-hang from them, spin, jump, and rocket-boost straight up. Make sure you do this from every pipe so you don't miss anything!