This gem is found in the level Hit the Road in the Hazardous Wastes. At the end of the level N. Gin will start to chase you and Crash will jump in a cage-like ball to escape. After the first short chase you'll reach a checkpoint with a handful of dustbins and a Nitro crate. Spin the dustbin on the left and a remote control will pop out. Spin that and a remote-controlled toy car will appear. Spin the remote control a few more times to make the car drive into the Nitro crate and the Green Gem will appear. Note that the gem will only be there in the Normal version of the level and not the N. Verted mode.