Little clear up here for translation, there are several distinct ‘types’ that I think might be lost in translation.
dust - pile of dust, 9 small dusts make 1 dust and 9 dusts make one block of dust.
pebble - the scrap left over from broken stone tools and meteorite iron/steel, looks like a pile of little balls. This is the one I propose can be made into gravel as it is in the form of gravel stones.
scrap - the jagged leftovers from metal working or broken tools, also including wood scraps here too. Would not be allowed to form a block because it is scrap and therefore non uniform.
stone - the rocks you find lying around and from breaking cobblestone. 4 of these combine to make cobblestone. This is where I think the confusion arises with Mr.CHEN and pebbles.
cobblestone - standard mined stone block.