[petewailes]

It's not uncommon. Most Roman mosaics were laid using tesserae made of of local stone (most commonly limestone, sandstone and marble, but varies by area), glass and ceramics. Even in the case of the ceramics, the colour was baked in at firing, so there's nothing there (in terms of colours at least) in any of the materials, generally, to degrade. If they got worn, you just got the same colour through the whole of the tessera, so it wouldn't show in terms of degradation of the pattern - you'd just get wear and tear on the materials themselves.