Round river rock are smooth, rounded stones formed and polished by flowing water. They vary from pebbles to cobbles and include granite, basalt, quartz, limestone, and other types. Colors include gray, brown, tan, black, white, and sometimes red or green. These hard, low-porosity stones are used in place of mulch and ground cover, used for erosion control, borders, garden beds, paths, water features, dry streambeds, and drainage (especially French drains) because their rounded shape helps water flow.