Not true. There may be some, but most mosaics use about 7-10mm thick stone tile. I have seen a lot of them... They last because they are covered, and the better preserved are often in dry climates. Archeological sites tend to cover them back up with a layer of gravel to protect them.

No, the pieces a mostly cubic. I dug heaps out the ground near Swindon. A big pipeline was going in and a ruined villa was near it. After a month of digging we were told to grab anything we wanted as it would be trashed in the morning. I got a few mosaic squares, a nail and a cow tooth.