Medieval Mosaics:
These may be divided into four principal classes:
(1) Those used to decorate walls and vaults, made of glass cubes
(2) Those for pavements, made of natural marble, partly in large shaped pieces, and partly in small tesserae
(3) Glass in small pieces, either rectangular or triangular, used to enrich marble pulpits, columns, and other architectural features
(4) Wood mosaics
It is perhaps partly owing to the great wealth of Northern Africa in marbles of many colors and of varying shades that the finest of all Roman mosaics have been found in Algeria and Tunis, especially those from Carthage, some of which have been brought to the British Museum.

The range of color in the marble tesserae is vast, and is made use of with wonderful taste and skill: there are three or four different shades of red, and an equal number of yellows and greens, the last color in all its tints being almost peculiar to this part of Africa, and one of the most pleasant and harmonious in almost any combination.
The beauty and attraction to natural colored gemstones is a fascinating study. Beyond the decorative pleasures, it is believed by many that certain gemstones and minerals have definite and stable properties that work either alone or in conjunction with others, to produce specific desired effects.