Pebble filter is the necessary cushion filter in all the different filter chamber,our factory produce natural gravel (was elliptical-shaped variegated) and mechanical processing pebbles (nearly spherical, pink and white),it has hard stone, anti-pressure resistant, is the best cushion filter material, the density of 1.85 t / m3, Mohs hardness 7.5, hydrochloric acid soluble rate of ≤ 3.5%.