With the blacklight, I've found a Master-like structure on clearies.
Sometimes part lighting up and part not could come from it not all being one color of glass. The blacklight has led me to discover that some which looked like red clearies actually had both yellow and red glass.
It's fun to maybe get a clue to a maker of a clearie, but mostly I think it's a sign that the makers didn't worry about consistency in their glass just as long as they were getting the basic clearie look that they wanted.
Whatever colorants they mixed into the batch may not have fully mixed.
Would be interesting to know if MK made their clearies on the same kind of machine which made their Rainbows and if they had glass feeding in from different places to do a basically single color marble. But I don't know.