Rock polishing grit is made of silicon carbide, a substance harder than most rocks. It is much the same as silica sand, a sand used in sandblasting with a primary component of quartz. Polishing grit comes in coarse, medium and fine grits, just like sandpaper. The 80 grit is coarse, like unground sea salt; 120 and 220 are medium grits comparable to sand; 500 is the finest and feels like powder. It is possible to make your own polishing grit with quartz and a lot of elbow grease. Be sure to make some of each grit, as you will need them all to polish your stones properly.
Place the quartz pieces into the cloth bag and place the bag on a very sturdy surface. Keep the quartz pieces close together in the bottom of the bag.
Smash the quartz with the hammer. It should bread into several large pieces.
Hammer at the quartz, aiming at one piece at a time to break it up completely. Shake the small pieces into the bottom of the bag and continue hammering until the pieces resemble unground sea salt.
Remove about a third of the quartz pieces. This is the coarse polishing grit.
Smash the quartz with the hammer until the remaining pieces look and feel like sand. Remove another third. This is the medium polishing grit.
Hammer the remaining pieces until they are the consistency of powder. This is your finest grit.
Use a sturdy canvas bag and don't hit an area too many times to avoid holes forming.
Keep hands well clear of the hammer at all times. Wear protective eye wear to avoid getting quartz dust in your eyes.