Cobblestones are stones that were frequently used in the pavement of early streets. "Cobblestone" is derived from the very old English word "cob", which had a wide range of meanings, one of which was "rounded lump" with overtones of large size. "Cobble", which appeared in the 15th century, simply added the diminutive suffix "le" to "cob", and meant a small stone rounded by the flow of water; essentially, a large pebble. It was these smooth "cobbles", gathered from stream beds, that paved the first "cobblestone" streets.
Note that cobble is a generic geological term for any stone having dimensions between 2.5–10 inches. A cobbled area is known as a "causey", "cassay" or "cassie" in Scots (probably from causeway).[source]