So, everyone has watched Monster Reunion and Same Old World, right? Well, if not, there will be spoilers.
So, remember this light thingy? The theory is focused around it.
It is from the Great Diamond Authority. In "Monster Reunion", Steven describes it as a song/sound. Now stop a little and think: Who in this show so far has an attack that is a sound that emits light?
That's right, the "cotton candy of the jungle"
Now, I'm not saying it was our Lion who was used. I'm saying that each member of the authority had their own lion. This would also explain why the pink one left: In Monster Reunion, Centipeetle drew only 3 diamonds, so Pink already wasn't involved.
It would even explain why Rose kept it secretive: So other gems who knew about the weapon being Lions don't see her with such weapon.
I made these as a representation of how they might have looked like:
(Images are not to scale)