Travertine grade is most usually deciphered based on the holes within the stone. These holes / pockets are part of the stone criteria and nature. Typically within the Travertine industry the fewer holes present in the stone, the higher the quality. Travertine Pavers with few holes is considered to be premium grade quality (or A Grade) while more porous stone containing more holes is considered to be commercial quality. Pricing differences apply based on this principle, while considering a few other factors in as well.
Also worth noting is that there are Travertine Tiles that come with a honed and filled finish. Filled travertine finish refers to the process of filling the holes. The filling is a solid color throughout the stone and can be seen quite easily. In these instances the differences between premium grade and commercial grade lies in the amount of filling used to fill the stone surfaces. This being said, a filled and honed travertine tile is considered of lower quality if it contains more filling than another stone.