The Yellow Gem is found on the level RUN IT BAYOU
At the very start of the level you will see some floating crates on the right side of the boat. Slide-jump to the floating crates & continue until you can jump to a moving platform. To the left of the moving platform are 4 more crates. Jump on top of the 4 crates, hold the jump button and do a double jump mid-air. At the very top, out of the screen is the Yellow Gem.