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Sand, by definition, consists of tiny, loose grains of material that are larger than silt but smaller than gravel. Much of the sand on beaches is made of eroded rock, like sandstone, but it can also be made of crushed seashells, parrotfish poop (which is actually very small bits of undigested coral skeleton), or the remains of calcium-containing sea creatures called foraminifera.

Source: Rachel Lense, 13th Jun 2023, 8 Granular Facts About Sand,  MENTAL FLOSS.