We know that, for example, a tribute including ten turquoise masks were sent to the Aztec capital from Oaxaca each year. Turquoise, a valuable non-transparent mineral, was used in a lot of Aztec art. Sometimes masks were covered in mosaics made of various materials. The mask itself could be made of green or black stone, wood, obsidian (a hard dark volcanic glass/stone), or even placed on a real human skull. Common mosaic materials were turquoise, obsidian, gold pyrite, coral, and shell. Sometimes the mask would simply have inlaid teeth and eyes.
Ceramic masks were more rare, but they were made. The mask represented by the drawing above is ceramic. That mask is from Teotihuacán, and is believed to be a death mask. The eyes would have been filled in at one time. It may represent the god Xochipilli. See this and more Aztec masks here.
What were they used for? y8
Aztec masks were used as ornaments, and were sometimes worn as part of a ritual, or in death as a death mask. They usually represented one god or another, and the Aztecs did have many gods. Like the artistry and materials, the Aztecs worshiped gods collected from a variety of cultures. A common type of mask would have snakes on it, a representation of the god Quetzalcoatl or perhaps Tlaloc. The masks generally then were used for worship of the gods, whether by being displayed in a temple or worn by a priest.
Below is another Aztec mask made of stone. Masks with closed eyes and an open mouth were usually representative of death. In this case, the wearer of the mask would appear to be wearing the skin of a sacrificial victim (the mouth is a hole), something that was done by the priests (see another version of the mask here) when sacrificing someone to this god, Xipe Totec (the Flayed Lord). Notice the ear spools, commonly worn by Aztec people.