What is the Roman Coliseum mostly made of?

The Colosseum or Coliseum, also known as the Flavian Amphitheatre, is an oval amphitheatre in the centre of the city of Rome, Italy. Built of concrete and sand, it is the largest amphitheatre ever built. The Colosseum is situated just east of the Roman Forum. Construction began under the emperor Vespasian in AD 72, and was completed in AD 80 under his successor and heir Titus. Further modifications were made during the reign of Domitian (81–96). These three emperors are known as the Flavian dynasty, and the amphitheatre was named in Latin for its association with their family name (Flavius).

Concrete is a heavy, rough building material made from a mixture of broken stone or gravel, sand, cement, and water, that can be spread or poured into molds and that forms a stonelike mass on hardening.