Well, stone cutting tools = $10 jewellers piercing saw, not a large investment i think you'll agree.
The stuff you want, if you decide to go the 'fill the cavity' route is called Inlace and for this check out woodturner's suppliers, you should find it cheaper than the link you posted. Turners use this stuff, mixed with epoxy, a hell of a lot and i must admit i'm thinking of using it for grainfilling one day.....don't ask me why, it's one of those 3am 'great idea' things
Jem