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in mosaic work, a small piece of stone, glass, ceramic, or other hard material cut in a cubical or some other regular shape. The earliest tesserae, which by 200 BC had replaced natural pebbles in Hellenistic mosaics, were cut from marble and limestone. Stone tesserae remained dominant in mosaics into Roman times, but between the 3rd and 1st centuries BC tesserae of smalto, or coloured glass, also began to be produced, cut from large slabs of glass that ranged from lightly tinted to opaque. These relatively fragile glass tesserae were used sparingly in floor mosaics to provide pure blues, reds, and greens that could not be found in the more durable natural stone; with the advent of wall mosaic between the 1st and 3rd centuries AD, however, glass tesserae of every hue were produced to constitute the major part of this decoration, stone being mainly reserved for floors. Glass was the major material for wall and vault mosaics of Early Christian and Byzantine churches, and marble and limestone tesserae were frequently used in the depiction of faces, woolen garments, rocks, and other objects that required a soft or rough appearance.