To answer a question at work about plasma physics I bought the cheapest plasma sphere I could find. We didn't get a conclusive answer on the original question, but playing with it brought up some interesting behaviour:
I asked Jan if he could explain this and he's done some digging:
jan: The globe is prepared by pumping out as much air as is practical. The globe is then back-filled with neon to a pressure similar to one atmosphere. If the radio-frequency power is turned on, if the globe is "struck" or "lit", now, the whole globe will glow a diffuse red. If a little argon is added, the filaments will form. If a very little xenon is added, the "flowers" will bloom at the ends of the filaments. says wikipedia
njh: ok, so the answer is xenon. But I was more interested in why. (And why does xenon make this happen and not neon?)
jan: sure, just a piece of evidence - the fact that it's caused by addition of another noble gas should tell something, but I am not sure what.
jan: my roommate says https://en.wikipedia.org/wiki/Ferrofluid
jan: it could be similar mechanism, the symmetry breaking
jan: so I talked to a guy who does plasma physics (tokamaks), he had an interesting observation
jan: the electrical field in the shape of the 'petal' is there even without the xenon, it's just xenon glows in right region, so the petal shows up
jan: the fact that the petal is very red supports this:
jan: if I was optimistic I would say that xenon has the lowest energy bands
I'll need to dig out my spectroscope and see if I can determine the precise wavelength.