Southern Products & Silica 420031 50 lbs 1.8mm-2.2mm GP 3 Rounded Course Quartzite Filter Sand. Our materials are hard quartzite, sub-angular sand and rounded gravel, free of clay, silt, iron, mica and other foreign matter. All filtering material is washed, screen-sized, washed and sized again according to current American Water Works Association B100 Standards for Filtering Materials and are NSF listed.