Rubble stone is irregularly-sized, rough stone which can be used for a variety of purposes, including rubble stone walls, fill, and stepping stones. People have been building with rubble stone for thousands of years, and it continues to be a popular building material in regions where there are ample supplies of rough stone. - http://www.wisegeek.com/what-is-rubble-stone.htm


rubble - http://dictionary.reference.com/browse/Rubble

3. rough fragments of broken stone, formed by geological processes, in quarrying, etc., and sometimes used in masonry.
4. masonry built of rough fragments of broken stone.