Answer by
LeheckaG (1826)
Grout for ceramic tile often has some sand in it.
Travertine stone is very porous and soft;
You usually should use an "unsanded" grout so that it does not mar the soft travertine stone finish.
Since travertine stone is very porous,
you should apply a couple layers of NON-glossy sealant to the travetine stone BEFORE grouting;
as well as afterwards.