Sand covers so much of the earth’s surface that shipping it across borders—even uncontested ones—seems extreme. But sand isn’t just sand, it turns out. In the industrial world, it’s “aggregate,” a category that includes gravel, crushed stone, and various recycled materials. Natural aggregate is the world’s second most heavily exploited natural resource, after water, and for many uses the right kind is scarce or inaccessible. In 2014, the United Nations Environment Programme published a report titled “Sand, Rarer Than One Thinks,” which concluded that the mining of sand and gravel “greatly exceeds natural renewal rates” and that “the amount being mined is increasing exponentially, mainly as a result of rapid economic growth in Asia.” ... Geologists define sand not by composition but by size, as grains between 0.0625 and two millimetres across. Just below sand on the size scale is silt; just above it is gravel. Most sand consists chiefly of quartz, the commonest form of silica, but there are other kinds. Sand on ocean beaches usually includes a high proportion of shell pieces and, increasingly, bits of decomposing plastic trash ... Sand is also classified by shape, in configurations that range from oblong and sharply angular to nearly spherical and smooth. Desert sand is almost always highly rounded, because strong winds knock the grains together so forcefully that protrusions and sharp edges break off. River sand is more angular. ... Aggregate is the main constituent of concrete (eighty per cent) and asphalt (ninety-four per cent), and it’s also the primary base material that concrete and asphalt are placed on during the building of roads, buildings, parking lots, runways, and many other structures. A report published in 2004 by the American Geological Institute said that a typical American house requires more than a hundred tons of sand, gravel, and crushed stone for the foundation, basement, garage, and driveway, and more than two hundred tons if you include its share of the street that runs in front of it. A mile-long section of a single lane of an American interstate highway requires thirty-eight thousand tons.