If you visit any mineral show or shop you will see lot of sheres and stone balls, these are two bifferent things.
I make sheres, my technic only for it, the balls coming from China.
In Hungary we are only 5 person who can make sheres, the main reason is we need few special tools for it.
What are these?
Cutting machine what enough good for cutt big stones
and heve a guide for paralel cutting
We also need an other machune with vertical axel for the steel cup what we use for sending. I use SiC powder 400-800-1000 grit.
First we select a good peace of stone, i will enough size for a sphere, i make 4cm of diameter shares only.
So, the stones must be biggr than 4cm to any way.
First cut a silmpe lap clost to the side of the stone pay attention you leave enough material for the paralel cut.
Next cut the paralel with the firs cut, if you cut 4cm of sphere you need 4,2mm of cube , when you finish with the paralel cut tunt the stone 90 degrees to the lap and cut the next sides all around the stone.
Now you have a nice cube, all side is 4.2x4,2cm Next cuts the corners, cut all of them off, pay attention, keep the size well, if you cut too much from one side you mast send a lot of time, and your shere will be little than 4cm,
After the work.
Next step the rough sending with a 80 drit of diamond wheel, all small corner must go out, in the end it you have a pre formed sphere.
For the fine sending you have two way:
One: Use steel cup with SiC, this way very long for a nice form it will be 2-3 hours!
The other way use diamond coated drills or cup, you can buy these in ready for use in the internet, the work with these tolls less than 20 min.
When you finish the fine sending you have a smooth surface sphere, you need only the polish.
I have for the polish two aluminium cau with felt inside, i use Cerium Oxide for polish, usually 5-8 min. enough for the mirror surface all around the sphere.