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Why are most of the deserts and beaches sandy? And why are most of these sands quartz?
--- Annette, Regis University
Quartz is relatively inert, making it the most resistant to chemical weathering of the common rock-forming minerals. As rivers transport weathered material to the sea (in keeping with the 3rd Law of GeoFantasy), most everything else is dissolved (in keeping with the 4th). Sand sized pieces of quartz are the most common solid material which actually makes it to the beach. It's important to note that the longer the river, the more time the earth has to purify the materials, and the "cleaner" the quartz sand on the beach (see the question on active and passive continental margins for more on this). Wave action keeps the smaller stuff - silt and clay - in suspension (it settles to the bottom farther offshore in deeper water where the energy is less), leaving the sand at the shoreline. The concentration of sand works in a similar way in arid lands, where the energy of the wind winnows away the smaller stuff, leaving sand-sized particles behind.
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