You are going to get a lot of polishing requests from people. And can your dad's sphere machine make spheres out of other materials? Trackball polishing and custom trackball balls could finance your next MAME cab!
Also, let us know how the stone sphere works- I was thinking about buying one for just that purpose, but was afraid the weight might make it sluggish. Also wondered if the hardness might hurt your hand. I'm SURE it would for Golden Tee ball slammers, but would it bother you in regular play, or when you play for a long time?