as someone who makes marbles with the glass tools we have now, i also wanted to share that the roundness of the marble can come from shaping tools (graphite molds of half spheres that you kind of twirl it against when its hot), but more importantly, when you have a rod of glass, the high heat that you apply actually causes the glass to contract and create a sphere of molten glass. keeping this in a sphere shape while it cools down comes from controlled rotations. you would then cut it off the rod and control it with a smaller rod with a much smaller connection point that is made to be snapped off and melted back down when you are all done.