Tesserae
There are many different types of tesserae used in both modern & traditional Mosaics. From smalti & marble, to broken tile & found objects, mosaic artists are limited by only their imaginations.
Smalti
Smalti (the plural of smalto) is the glass tile used in classical Byzantine mosaics. It is made by mixing glass and minerals (usually metal oxides) to produce colored slabs of opaque glass that are cut by hand into rough rectangles. Even within a single batch, Smalti tiles vary in thickness, shape, size and color.
For more information see Wikipedia:Glass tiles and the excellent smalti page at MosaicArtSupply.com.
