The term road metal refers to the crushed rock used for road beds, surfaces, 'all-weather' walkways and paths, foundations, and railway embankments, among other things. The use of road metal dates back to antiquity.
The word metal comes from the Latin "metallum" and the Greek "metallon" meaning "mine, quarry, ore, metal". The reference to crushed rock thus becomes more apparent. It was more recently pioneered by Scottish engineer John Loudon McAdam in the late 18th century.
The term is relatively rarely used today, with "gravel" being more typically used and understood. One of the places where it is still in common usage is New Zealand.