30 minutes ago, keychris said:
Leaf gelatine should give you a very clear jelly.
I honestly don't think that a phone has enough vibration to make a jelly wobble back and forward, I feel like the frequency is too high, I have no proof other than a gut feeling!
I think you're right - maybe if it was an extremely hard-set jelly in a thin layer. I imagine something along the lines of a slow, steady pulse would work.
ETA: Steve Mould looks at the resonating frequency of a water balloon here, and it's really slow. I imagine jelly would be at least fairly similar.