It is hard because of the silica which also happens to be the most abundant mineral on earth. When rocks are weathered and eroded they break down into smaller pieces of rocks and minerals. These materials, which are called sediments, may eventually be carried to a new site by wind or water and deposited, often in the sea or in river beds. The sediments are deposited in layers which become buried and compacted. In time the particles are cemented together to form new rocks, known as sedimentary rocks.
Sediments take various forms.
The particles are sometimes so small you can barely see them. The smallest particle is clay. Next in size is silt - then sand, pebbles, cobbles, and finally boulders. Sandstone is a sedimentary rock composed of grains of sand which have been cemented together by other minerals. A conglomerate is a collection of smaller rocks which have become cemented together.