Sandpaper is a generic term although they used to make sandpaper with sand. It is not really suitable as the sand has been weathered and so is not as sharp as crushed glass. Garnet (6-7 Mohs) is a common low value gemstone with good characteristics for sandpaper but is now seldom used as other materials such as Aluminum Oxide (9 Mohs) and Silicon Carbide (almost as hard as Diamond which is 10 Mohs) are more common.
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