Some levels are marked with two gems. To get the second gem, you must activate the colored platform in that level. The activation for that platform is always in a separate level!

7: Hang 'em High - get the gold gem
11: Dyno Might! - has gold platform

10: Tomb Wader - get the blue gem
16: Sphynxinator - has blue platform

12: Deep Trouble - get the red gem
4: Boneyard - has red platform

13: High Time - get the purple gem
9: Tomb Time - has purple platform

23: Flaming Passion - get the green gem
19: Future Frenzy - has green platform

Crash Bandicoot Warped Walkthrough