Rock The Casaba is probably the hardest box gem on the planet.
The bonus is impossible to do with the nitros. So follow these steps.
1. Finish the magic carpet bit and then fall in the hole. Get the checkpoint.
2. Finish the level down there (you might get a gem shard) and hit the nitro detonator.
3. Go back to Rock The Casaba and get all the boxes you missed. When there is two near one of those fire people, leave ONE! (extremely important)
4. Do the bonus without the nitros (still quite hard, but possible)
5. Go back to the end of the level. (NOTE: If you didn't leave the box in step three, you will not be able to finish the level. I got it the first time I wanted to play and I had to start all over again cause I couldn't reach the end of the level)
I Give only ONE Warning!
6. Enter the porthole, remembering to collect the gem.