A pebble is a clast of rock with a particle size of 2 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered larger than granules (2 to 4 millimetres diameter) and smaller than cobbles (64 to 256 millimetres diameter).
Porcelain and ceramic tile are both are part of the larger category of tiles we can call ceramic. For modern tile specifications, it is more a case of reverse-naming, whereby manufacturers take tiles that have certain qualities and then assign the ceramic or porcelain titles to them.
Most commonly, travertine tiles are used indoors for bathroom flooring, but they can also be used in kitchens, outdoor spaces, and to decorate walls, ceilings, and countertops. Before being polished and honed, the tile provides a high friction surface suitable for use outside to prevent slipping.