This gem in unlocked at the beginning of the "Run it Bayou" level in "Mosquito Marsh". At the beginning of the level, BEFORE getting on the boat Crash can drive, you will notice 9 crates to the right in the water. Perform a slide double jump to this area and then cross the 9 crates to the other side.
Perform a slide double jump to the moving platform and let it take you to the area on the left with the 4 crates. The yellow gem is above this crates, so do a high jump above them to grab the yellow gem.
Note: You must complete the level for the achievement to unlock.
In case any of you like a video guide as well, please see below a video guide, courtesy of Maka91: