I would use a soft brush and depending upon the condition of the stone I would use an alkaline based cleaner or a neutral cleaner to clean the surface.
The soft brush will not hurt the stone and will clean the cleft areas well.
John E. Freitag
The Stone and Tile School
Customer has 5000sf of recently and very nicely installed quartzite tile around a heavily use pool. they need a maintenance program but we have red dirt mineral staining here. most restorers are using abrasive brushes and smoothing over the natural rough cleft finish . besides AB that can damage quartzite how would one maintain this without damageing the stone? Any Ideas?