This is what I use to clean my rock. A simple nail brush. I have seen so many people clean their rocks in different ways and they complain about the dirty ice if their rock picks. I have see so much dirt fly off the rock when I clean my stone with this brush. I don't believe others get their stone clean when they use their hand or broom. Have you ever heard of a soft pick this is when the stone just seems to want to start to curl prematurely. If you do not get the lint out of the granites fine crevices you might get a soft pick.
I also use this brush to clean the Ultimate Broom Heads that I make until recently. When I was using a white broom head to see how much visable dirt was on the ice. I was suprised to see all of the dirt on it, especially the tiny fibers from regular broom heads. These tiny fibers did not brush off as easily as the other dirt. But what I did find was if I cleaned it on the carpet on the bench or the carpet on the end boards it came perfectly clean. When we now clean our brooms we no longer clean them on the ice surface we go to the carpet. Why put the dirt back on the ice just to remove it again and complain about the DIRT.