Diamond wire saw beads are normally used in giant saws to cut huge blocks in stone quarries or to shape large stone sculpture.  I recently purchased some and made sphere grinding cups.

I think this is the best way to make larger cups.  This technique allows the beads to be turned on the wire to provide new surfaces to be exposed.

  Smaller cups can be made by just glueing the beads in place.  This could be as few as 3 beads which would be suitable for spheres as small as perhaps 3/4".