Originally Posted by Ampchan

might be like the rainbow medals from one of the old weekly gachas... they look like they exist, but the game crashes when you try to use them.... idk tbh... consider yourself lucky if they work still...

The stage select screen is the only place it shows 1 gem as the reward. It's looking like incompetence might have won out over fixing beneficial bugs. Previous instances of ghost-loot were fixed by a refresh but these gems are remaining. I'd try using them if they hadn't nerfed the gacha or I didn't have more than 50 gems before.