Sandstone is a sedimentary rock composed mainly of sand-sized mineral grains. Sand grains can vary in size and composition, but they are typically composed of quartz, feldspar, and other minerals. The grains are cemented together by silica, calcite, or other minerals.
The surface of sandstone is typically rough and uneven. This is because the sand grains are not completely cemented together, and there are often small gaps between them. The gaps between the grains scatter light and prevent sandstone from forming a clear and reflective surface. Because of this inherent trait, it cannot function as a suitable, practical mirror capable of forming accurate specular images like conventional glass mirrors.