Frankly I wonder if the "jelly" inside was ever real or if it is made like most "flowers in glass paperweights."
Almost every flower or animal I've seen in a glass
paperweight [versus one of resin - which works same way as amber encasing insects]
turns out to be a sculpture of colored glass which is then encased in more clear or tinted glass. Bubbles in a glass or resin paperweight are like "inclusions" in diamonds - they reduce the value. Most glass factories like Murano in Italy destroy the "seconds" to maintain the collectible value of the remainder (like DeBeers sends poor diamonds to industrial use.)
When examined under high magnification, the cell structure of so-called "flowers and plants" contained in glass paperweights is found to be absent. I suspect similar physical "details" of jellies may also be missing. When I was in college nearly half a century ago, I recall looking at slides of hydra and jellyfish tentacles under magnification and being able to discern the nematocysts or cnidocytes
(stinging cells) clearly present in the tentacles.
If the answer I have provided is the correct one, I apologize for destroying the mystery, but perhaps it will give a deeper appreciation for the art. I feel like Toto pulling the curtain away from the Wizard.