Ok I have a few minutes The rock that I have cut 6" spheres from have weighed 50lb or more. I spent $300.00 on the sodalite rock, but I got a lot of slabbing material from it. The price of the rock can vary from $1.00 a lb to ? I am able to collect a lot of rock where I live.
You can make a sphere by hand but it is a lot of work. if you search the internet you can find better instructions than I can Give you.you will need some kind of motor drive (100 to 200rpm is best) with a spindle that you can attach a 3/4' pipe thread male adapter and a steel pipe adapter from 2" to 3/4" , a brass 2" pipe 90 and a short piece of pipe for a handle. Cut a cube of rock (3" ?) and than cut off all the corners and hand grind it to a some what round preform. Start with 90 grit wet slurry and put the preform between the two pipe fittings and start grinding, rocking the pipe fitting around to keep the preform rocking and rolling always in motion keep it wet and add grit as needed, when round clean up everything one piece of grit will ruin everything if there are any cracks or pores you will have to clean the grit out of the rock, progress thru the grits always cleaning between grits. Polish you can attach felt or some other material to hold the polish.
a softer rock like serpentine or travertine oynx would be easier to do. or spend a bunch of money on a sphere machine Or You can buy a sphere from someone that makes them