The Sahara Desert was at one point, underwater. Sand is a non-renewable resource over human timescales, and sand suitable for making concrete is in high demand. Sand is a particle between gravel and silt in size. The fish bite and scrape algae off of rocks and dead corals with their parrot-like beaks, grind up the inedible calcium-carbonate reef material (made mostly of coral skeletons) in their guts, and then excrete it as sand. This made it very large. This separates things by size, and the Sand dunes occur throughout the world, from coastal and lakeshore plains to arid desert regions. Sand is just weathered rock, if you look at a map of the sahara there are quite a few scattered mountains and highlands in the desert. ELI5: if sand at the beach is made by the ocean breaking down rocks and shells at the beach, where does sand come from in a desert? Natural processes are diverse. Desert sand, although plentiful, is not suitable for concrete. Sand in reality is many very small rocks. Sand is tiny fragments of rock. When rock wears down you get smaller bits of rock, or pebbles, and when they wear down you get even smaller bits and eventually you get down to sand, or silica. 50 billion tons of beach sand and fossil sand is used each year for construction. The whole thing didn'e emerge at once. The accumulation of windblown sand marks the beginning of one of nature's most interesting and beautiful phenomena. The reason it ends up washing up on the beach is that the sea or wind can move sand around very easily while rocks are more likely to stay put. Note that just because there is lots of sand, that doesn't necessarily make something a Desert. It was a beach that simply followed the shore as the water moved away. What may seem like a fairly uniform material, ends up being very different when you look closely. That is where the sand (and the original silt and clay) came from originally, weathering of rock, and the sahara is mostly exposed rock, with sand only making up a minority of its surface. The famous white-sand beaches of Hawaii, for example, actually come from the poop of parrotfish.