The Colosseum uses several materials. The main one is travertine, a limestone, marble, for decorations, tuff, volcanic rock, brick, and lime for links. We also used metal, mainly bronze, to bind the stones together.
Travertine is a sedimentary limestone, it is found where once there were rivers, springs, lakes. The Roman region is rich with such stones. It has the characteristic of being beige and particularly resistant. The one that was used to build the Colosseum came from the town of Bagni di Tivoli, formerly Tibur. It is the majority stone in the Colosseum, the basic material.
Most of the Colosseum's decorations were in marble, but they have all but disappeared, having been reused in the construction of other buildings in Rome. The entrances of the cavea were decorated with marble pieces as well as the balustrades. This stone has also been used as a seat for the first 3 rows, those reserved for the social class most affluent. Outside, the columns had marble capitals, and some columns were also in this material. Finally it was also used in different places: For eardrums, friezes, etc.
It seems that marble has been used since the construction of the Colosseum, but it is the subsequent renovations that will add. It is often that we will reuse parts, from the third century. Thus some marble pieces were previously used for other constructions before finishing here.
The Colosseum also uses metal in its structure, but nowadays it has disappeared. It was mostly bronze, but iron was also used. The metal, once heated, was stretched into a bar and then curved into a U shape. Adapted to holes intentionally dug into the stones of the facade, these structures served as agraphs to help keep the Colosseum upright. The first fires got the better of these agraphs which were gradually recovered to be melted. Nowadays we only have the holes in the stones, vestiges of this originality in the construction.
Tuff is a volcanic rock formed of slag that has agglomerated with moisture. The region of Rome is rich in tuff, there are different varieties, each having its color. This stone is not especially dense, used in construction it brings a little elasticity. The tuff that was originally used was not fire resistant so the Colosseum was badly damaged during the first fires. The 3rd century repairs were made with a variety of fire-resistant tuff ... which did not prevent the monument from burning several times!
Brick is a basic piece in Roman construction. It is made of clay mixed with water, straw or pozzolana. Once cooked it could be stacked with mortar to raise walls that did not require a great resistance to crushing or as tiles. The interior walls of the Colosseum use large-scale brick.
Lime is a product of the burning of limestone at high temperature, the product thus made is called quicklime. It is enough to add water to obtain a viscous mass usable in architecture, it serves as a binder for the cement. This is how the Romans made their cement.