For me, I take them on a shop that cut and sell marble and granite. They always do it for free :beer1:
For lapping, I have some 1.5m/6ftx20cm/8" pieces of sandpaper. Piece of cake to flatten when they are nicely cut.
For finding out what rock has potential to be a hone, try the spoon trick. Rub a somewhat flat piece of stone with the back side of the spoon. If there are scratches, the stone can cut.
Something else I have observed; many times, when there are lines of quartz in the same rock, it's not suitable, even if the piece you have doesn't have any of those lines. They have inclusions that many times are not visible. If you go far from those lines, there might be some hope.