A boulder is a fairly large rock. One can easily imagine a location – some have seen one – where a large number of boulders are present.
The next size of rock down from a boulder is a cobble. Typically cobbles are about 8" in diameter, though they come larger and smaller.
Next item down is the pebble. And a typical rocky shoreline is composed of pebbles. There is a sedimentary rock composed largely of pebbles and cement called conglomerate.
Then you get to gravel, particles smaller than pebbles but clearly individual rocks which can be seen at a distance.
Silty soil consists of fairly fine particles, generally with humus composed of decaying and decayed vegetation mixed in.
Getting even smaller than silt, we have clay, which consists of extremely fine particles, nearly always with humus.
Now, fill in the gap. The missing category of particle is sand. When rock is ground, it turns into coarser or finer particles, ranging from boulders down to clay.
Typically, sand is composed of silica and feldspar, though other minerals can stop at the sand level of coarseness. White Sands N.M. is composed largely of sand-size gypsum particles.
The ultimate origins of sand are in granite-type igneous rocks. (Someone will have to identify the various rocks that can act as sources.) But most sand over the millennia has been compressed and cemented into sandstone, and most present-day sand has been eroded from sandstone.