Travertine is formed by a process of rapid precipitation of calcium carbonate. It is often formed in the mouths of hot springs and it is these hot springs which create the holes seen in Travertine. It is important that these holes are filled, especially if the travertine is for interior use. The holes can be filled during production using a resin, or they can be filled on installation using tile grout. It is my opinion that filling is better off done during installation as this allows for a more authentic looking finish.