It is the material element of the mosaic, in the traditional opus tessellatum. It is usually a cube or parallelepiped of marble, glass material or stone, of varying sizes. The sides can range from 2 cm up to a few millimetres. The tessera is modelled by hand by the mosaicist with a special hammer (martellina) and a hardy (tagliolo).

The martellina is a hammer with two cutting pens: one is suitable for marble and the other is for glass and smalto.
The hardy resembles a chisel, driven into a wooden block, and it is the counter-cutting of the hammer.

It is the way in which the tesserae are laced to the surface, one next to the other, to determine an image or effect. The tiles can follow a linear pattern, split or break it and create new rhythms and patterns.

This is the distance between one tessera and the next one. It can be minimal, not revealing the bedding mortar, or leave more breathing space and uncover the surface to which the tesserae are embedded.

Since the era of early Christian-Byzantine wall mosaics, glass tesserae and gold tesserae were set at various different orientations and inclinations. This allows them to capture light and radiate it, creating amazing reverberations and a sense of surface movement.

A preparatory drawing on paper which, when transported on the tesserae bedding surface, serves as the outline for the execution of the mosaic. The cartoon is very important, especially for the copies of ancient mosaics, because it showing the colours and tesserae on a 1:1 scale.