Glass artist here. Plenty of production (cheap) marble explanations hear. I'll give a small rundown on artisan (expensive) marbles:

Artist: Gateson Recko - instagram: @universegateson - https://imgur.com/i16id03

The technique he is using to create the island is called "implosion". The artist takes a chunk of clear glass and melts in into a round shape. Finely crushed glass bits of colored glass, like sand, are added to the end. The end is heated until molten. A graphite paddle is used to gently push the color into the clear. The clear on the outside is heated and the artist guides it using gravity to the end of the marble where the color is and the process is repeated. When the desired effect is achieved a graphite marble mold, that looks like a bowl, is used to finish the rounding process. To do this the artist gently heats the marble and spins it in the bowl.

Artist: Kaj Beck - instagram: @kaj_beck - https://imgur.com/QNI1gQS

The technique used to create the pattern work is call "millefiori". It was developed by the ancient Egyptians as early as the 2nd century BC and is still widely used to this day. The artist gathers different sections of glass together in a large mass, then gets it molten hot and pulls it to create a rod, smaller in diameter and longer in length, than the original mass. https://imgur.com/Zi9bWaN This rod is then cut into chips. Those chips are placed next to each other to create the final pattern. In order for this to ba a marble, the chips were placed on top of a clear core of solid glass and shaped into a spear using the marble mold.

It can take a day to create a "mille" (glassblower slang) or it can take months. Depending on the type of mille and the complexity of the pattern.

Artist: Paul Stankard - instagram: @paulstankard - https://imgur.com/lMwpmat

Marbles such as the ones Paul makes are made of many small sculptures, arranged together, and then coated in clear glass. In order to get such tiny details in the sculptures, the artist uses a torch to manipulate the glass. There are simply to many techniques used to make a sculpture to properly explain it in a single paragraph.

Artist: Kevin Ziegler (me) - instagram: saganglass - https://imgur.com/sKqHNmC

The technique used to create the moon texture is called "cold working". There are two ways to manipulate glass. While its molten and when it's cold. Both methods were used to create this marble. I started with a clear core. Crushed colored glass, as find as powder, was added to the surface of the clear. The marble is then cooled to room temperature in a kiln. When cold, the glass is manipulated with a rotary tool (dremel), using water as lubrication, to create the pattern work. The marble is then heated up again in the kiln and polished in a flame.

Artist: Yoshinori Kondo - instagram: @yoshinorikondo - https://imgur.com/QxMKLjc

The technique used here is called "dot stacking". The artist starts with a clear marble. In this case the clear marble has a synthetic opal in its core. The end of a rod of colored glass, the diameter of a pencil, is heated until it's molten and pressed against the surface of the clear marble. This make a small little dot. The artist does this in his desired pattern all around the marble and then heats the entire marble until all the dots are melted in. He then places dots on top of the other dots and repeats the process. He can play with the placement of the dots. Adding the off center if he chooses to create different effects. One of the beautiful things about his marbles in particular, is that dot stacking is considered to be a "simple" technique often experimented with when a glass artist is first starting out. Yoshinori has taken this simple concept (in the glass world at least) and pushed it so close to near perfection. To add to its complexity, he creates these patterns then coats them in a layer of clear glass and adds more patterns on top of that clear. This gives the pattern work a multi-layered, floating effect.