Here is a photo of a mosaic made by a student of mine which he then polished up at home. He used a rough grinding stone to level the mosaic off and then used polishing pads to bring the shine up.
This is no different to the way in which the Romans would have finished off their floors. Grinding stones to level off and fine grit with smoother stones to bring the polish up. In this way you can end up with a surface like glass.
Here is a photo from a mosaic in The British Museum. It has been said that the Victorians gave these a clean up but even if this is true it would be the same finish as the ancient mosaicists would have aimed for.