Gravel and sand are used for road surfaces, and for cement and Tarmac. Mixtures of gravel and sand mixed with various crushed material, are known as aggregate in the trade. This is used for drainage in the basement of roads, and other purposes.
Natural gravel comes from ancient beaches which have not consolidated (turned into solid rock). In effect, beaches from (for instance) the Jurassic are dug up in quarries. The material is then sifted into size categories. The gravel and sand is then sold on to engineering companies. Most natural sand and gravel is silica, SiO2, in its chemistry. This is very hard-wearing.
The open-cast mining and processing of gravel, sand and aggregate is a huge world-wide industry, sometimes called "conglomerates".