A glass-like ceramic, popular in Egypt around 2500 BC.
Faience (pronounced similar to "seance") is a ceramic matieral dating as early as 3500 BC that was a precursor to (and an important step toward) glass. It produced by firing some non-clay minerals, predominantly silica and quartz (that's right, the same ingredients in semiconductor circuits!) all sintered together, with additives to change its appearance. Most often, high amounts of copper are present, giving the faience a blue-green color that was the sign of wealth in the desert -- it looked like water. Other colorants include lead, cobalt and antimony. (NB: don't drink out of faience cups.)
Faience was typically used as an outer coating for stone-carved statues and art objects. Many figurines, beads, cups and amulets were made of (or decorated with) the material, including the relatively infamous funeral ushabti (see that node for more information).
The Egyptians used two words to describe faience, "tjehnet," and less often "khshdj" (the latter being synonymous with lapis lazuli, a substance with which faience shares much of its appearance). The most famous (and formidable) faience project was Pharaoh Zoser's tomb, widely accepted as the model for most Egyptian pyramids. Over 35,000 faience tiles were made to line the walls of his tomb, making aqua-colored hallways which created an ambiance similar to modern-day tile bathrooms or swimming pools.
It's not clay, it's not glass, but it's ceramic, and it looks damn cool: It's faience.
Parsons, M. Egyptian Faience (http://www.touregypt.net/featurestories/faience.htm)
Vandiver, P., Research Physical Scientist at the Smithsonian Institution
Infinite Burn (for vocabulary help)