Sedimentary rock.
“Sedimentary beach stones are formed on shores or in the water by the weathering of older stones. These bits and chips of older stones settle to the ground on layers along with the remains of organic materials, like parts from plants and animals. Through time they harden into compact sediment. Fossil stones found on the beach are this kind of rocks. Other examples of sedimentary rocks and beach pebbles are: sandstone, shale, conglomerates and breccia”