The grade of travertine is most of the time determined based on the holes within the stone. Holes within Travertine stone are part of the stones nature and criteria.
Within the travertine industry today, the rule of the thumb is …the less holes there are within travertine stone, the higher the quality. Travertine pavers with barely any holes in it are considered A Grade or Premium Grade quality while porous stone with a lot of holes in it (looking like a sponge) is considered commercial quality. Obviously price differences apply.
Also keep in mind that there are Travertine tiles which come with a honed and filled finish. A filled travertine finish indicates that the holes were filled. Since the filling is always the same color throughout the tile it is fairly easy to tell apart the filling. In these cases, the difference between Premium Grade and Commercial grade lies in the amount of filling used. A honed and filled travertine tile with a whole lot of filling is considered of lower quality compared to a tile with minimal filling.