The link below takes you through to a BBC documentry on the Roman Empire (on Youtube). In one section, on tools found in a quarry, one of the hammer heads looks the closest i've seen to a type that could be used to cut tesserae in the same way we work now. All the other hammers I've seen found in stoneworking sites have been what's known as scabbing hammers, pointed at both ends. Look at the hammer which is the third one away from the camera, although it has a normal, blunt end it does have a flat end too.
There are two arguements against this though;
1. Would they have been cutting tesserae in one of the quarries or waiting to get the material from building sites/sculptures workshops?
2. The quarry there is Carrara, white marble whereas I've always been told that an off white (ie Botticino) is used in mosaic work as pure white has a very 'flat' look to it whereas something like Botticino, with its different shades ahs more depth. Something to look into.
Go to 21:53 https://www.youtube.com/watch?v=h61BZ-O9Wuo