A mosaic is a picture or pattern created with many small bits of glass, stone, ceramics or (in rare cases) gemstone. In ancient Greece mosaics were created by using natural stones, often from riverbeds, but by the Roman period they were created out of pre-cut cubes of glass or stone known as tesserae.
Mosaics were often used to decorate floors, walls, and the insides of domes. Constantinople was famous for its many notable mosaics, and St.Peter's Basilica in Rome is decorated in many mosaics, including what appear to be the paintings behind altars. Mosaics can be fixed to other flat objects such as a table but can also be applied to three dimensional objects such as vases. Mortar is used to secure the mosaic material to the surface and grout is used to fill the gaps between the pieces.