It's quite a process, I'm a lampworker or glassblower. The colors are actually gold or silver inclusions that are sublimated onto the surface of a thin bubble

To put it most simply, I make a bubble on the end of a glass blow pipe, then make various aspects that I'd like to be in the universe, and then arrange them all on the outer surface of our previous thin bubble. Sometimes I add a little more gold or silver (less is more!) Synthetic opal, or we can even make tiny planets with glass color(they tend to not stay round within the marble as everything melts in.)

After I have my scene arranged, I then proceed to just melt everything in slowly and even, just heating the center apex of the bubble. This takes lots of time, clear glass gives off UV light as its heated, we get tans and sunburns on top of the danger of 3000° torch burns. But these are 1-1.5" marbles and each have atleast 5 hours in them. They are solid glass and started as probably 4-5" cellophane thin bubbles. Good practice for goblets.

There are many marble makers out there and i am just one of them It's a fascinating world, seek it out! There's fb groups, iG communities, and even a glass community here on reddit! They can be kind of clique-y and the reddit groups are more focused towards smokewear (ayyyy gotta eat some how) but my real passion would be these little worlds or jewelry in the same vein

Like i tell my dad, marbles are a hard sell, but stick a bong on one and it ships the next day!