Florentine mosaics are different from other types of mosaic work such as Byzantine or glass mosaics.
With Florence mosaics, the cut pieces of stone fit together with each other in such a way that you can't tell that the finished work is, in fact, made up of many little bits of semi-precious stone.
This perfect joining of the stone pieces is called 'commesso'.
With other mosaic styles the spaces between the tiles/pieces are quite obvious, indeed, are meant to be seen.
How is this done?
With painstaking work and enormous patience.
Each piece of stone - often minute in size - is carved, shaped, filed, measured, and re-filed until it meets perfectly with its connecting piece.