Nobody agrees how big a stone can be and still count as gravel
Geologists usually call any loose, rounded rock fragment over 2 millimetres gravel, with no upper limit. American soil scientists stop at 80 millimetres, the German scale runs to 200, and the US Army Corps of Engineers uses sieves. The humble material is also a giant industry: America produced some 960 million tons of construction sand and gravel in 2020.
Most gravel starts as bedrock breaking apart through weathering. Quartz dominates because it is hard, chemically unreactive and has no cleavage planes to split along. Since few rocks have crystals coarser than about 8 millimetres, most pebbles are made of many mineral grains rather than one. Rivers round the fragments quickly, often within a few tens of kilometres of where they broke off, and drop them as bars in channels, in alluvial fans, along coasts and in fast-flowing deltas. Angular pieces in the same size range are sometimes called rubble instead.
Definitions multiply because different trades need different things. The Udden-Wentworth scale used by many US geologists splits granular gravel, 2 to 4 millimetres, from pebble gravel up to 64. The ISO 14688 engineering standard grades fine, medium and coarse bands running from 2 up to 63 millimetres. American builders separate natural gravel from crushed stone made by machines, and the US Geological Survey tracks them separately too. A cubic metre typically weighs about 1,800 kilograms.
Commercially, gravel is everywhere. Almost half of construction sand and gravel becomes concrete aggregate, and much of the rest goes into road bases, asphalt surfaces and fill. In 2020, sand and gravel together made up 23 percent of US industrial mineral production, worth about 12.6 billion dollars. Porous natural gravel beds let water flow through very freely, sometimes over a centimetre per second, which makes them valuable aquifers. Even so, in the eastern United States machine-crushed rock is already pushing the natural kind aside, and recycled material is gaining ground.
Gravel has colourful varieties: pay gravel rich enough in gold to pan, and pea gravel sized like garden peas for paths and aquariums. Plants struggle on gravelly soil, which holds little moisture or nutrients. Cemented into rock, gravel becomes conglomerate, less than 1 percent of sedimentary rock. And wind-built gravel megaripples in Argentina may have counterparts on Mars.
Source: Gravel
