Ok but on the Island Adventure episode the invisible monster has a purple gem on its chest. So far there hasn't been any gems that are the same. Granted we have only met a few and there are not infinite places on the body or colors in the rainbow but still I think that maybe something.
Well, the gem on the invisible monster in "Island Adventure" looks blue to me:
while the 'inner' part is slightly purplish, its not really the same kind of purple Amethyst's gem is, its more like the kind of purple you use to shade light blue. So I'm doing think its related.
It is the first chest gem placement we've seen on a monster that matches the placement of one of the crystal gems, though. The ice gem from "Monster Buddies" had a forehead gem. But a lot of the monsters don't have humanoid shapes so its hard to tell where some of the gems would be on a humanoid