I was trying to avoid spoilers in my comment, but I don't think I can say it without adding spoilers, so...
[SPOILERS]
In the fourth level (the one with the wall of death), there's a bunch of gems with messages around them telling you not to collect them. As far as I can tell, it's not possible to grab all of them (though I could be wrong), but it is possible to grab some. The line between was is collectible and isn't is kinda fuzzy there, which makes defining a 100% kinda tricky to me.
[/SPOILERS]
Also, since I forgot to comment this last time and I try to make comments about this when it's relevant: The gems aren't very colorblind friendly. The three colors don't stand out from each other very much (with the first two being particularly bad for me personally). It's less of an issue since differentiating between them doesn't have any effect on the mechanics of the game, but it could make figuring out what you're missing for 100% harder to figure out. Next time I'd recommend either choosing more contrasting colors or making the gems distinct shapes.