Fluid Systems Gravel and water filter pebble is composed of sub-angular, hard, durable. and dense grains of predominately siliceous material. Extracted from a clean glacial deposit. Fluid Systems Minerals Gravel and Pebble''s physical properties make it among the finest available in the world for water filtration applications. Fluid Systems water filter gravel is washed, kiln dried, and screened t o meet exact in g specifications with strict adherence to quality control.