Short answers, pulled from the story.
Gravel is a loose aggregation of rock fragments, most of it derived from bedrock that disintegrates as it weathers. Quartz is the most common mineral in gravel because it is hard, chemically inert, and lacks cleavage planes. River transport rapidly rounds the fragments, often within a few tens of kilometres of their source outcrops.
Gravel is classified by particle size range, spanning granule- to boulder-sized fragments. The Udden-Wentworth scale divides it into granular gravel and pebble gravel, while ISO 14688 grades it as fine, medium, and coarse across ranges of 2 millimetres, 6.3 millimetres, 20 millimetres, and 63 millimetres. Many geologists simply define gravel as loose rounded rock particles over 2 millimetres in diameter.
Almost half of all gravel production is used as aggregate for concrete. Much of the rest goes into road construction as road base or road surface, sometimes with asphalt or other binders, plus construction fill and many minor uses. Porous gravel deposits also act as important aquifers because of their high hydraulic conductivity.
Gravel is a natural material, while crushed stone is produced artificially by mechanically crushing rock, a distinction the American construction industry and the U.S. Geological Survey both make. Crushed stone can be made from granite, limestone, dolomite, and other rocks, and it is already displacing natural gravel in the eastern United States.
One cubic metre of gravel typically weighs about 1,800 kilograms, and one cubic yard weighs about 3,000 pounds.
In 2020, some 960 million tons of construction sand and gravel were produced in the U.S. Sand and gravel together made up 23 percent of all industrial mineral production that year, with a total value of about 12.6 billion dollars.
Types of gravel include bank gravel, bench gravel, crushed stone, fine gravel, lag gravel, pay gravel, pea gravel, piedmont gravel, plateau gravel, and shingle. They are distinguished by composition, origin, and use, with names like pay dirt for gold-bearing gravel and pea gravel for clean stone the size of garden peas.