I made my first marble mold using a cheap set of round bottom router bits, using a drill press but I found you must clamp the graphite block in a vice, etc to stop any movement (my first mold was not round because it kept moving)
Safety note; you must have dust extraction when using power tools on graphite – I used a vacuum cleaner & and wear a dust mask.
Graphite is soft & easy to machine, a cheap set of router bits under $20, sizes 6 – 32mm (1.25”). Larger ones start costing more & biggest I’ve seen is 2”
I wanted bigger marble molds, first considered making large ‘router bits’ from something like large washers, welding on (or brazing) a shaft & sharpening a cutting edge. (If you go with this idea, be aware of the danger of spinning large cutting tools!!!)
I ended up making a concave cutting attachment for my metal lathe, this lets me make any size marble mold from 50 to 125mm (2-5”).
An easier way is to use a wood turning lathe (if I had one) & use bowl turning methods.
I have also made a set of graphite push, using the lathe. They could also be made with a metal file. Graphite can be smoothed with fine sand paper, remove graphite dust by washing with water & cloth.
Peter