Cracking is the most common cause of damaged grout.
Marble floor without grout.
As you walk on tile floors the grout becomes weaker and can eventually start to show unsightly cracks.
If the grout lines still do not come clean it may be necessary to remove the stained grout and regrout the marble tile grout to restore the grout to its new looking condition.
Acrylic latex grout is a good alternative but it will require sealing.
Repairing cracks with caulk.
The best thing to do is use the smallest grout line your particular tile will allow and get a grout that closely matches the tile.
Using the flat side of your trowel skim coat the back side of the marble or travertine with a thin layer of thinset.
It is a relatively easy project and it can be done quickly allowing you to make a dramatic improvement for a small investment.
If my hunch is correct regarding the sizing of the tiles then try and have the the grout lines on the base board hit the mid point on the floor tile.
Grout works to prevent moisture and detritus from entering and getting stuck in the grout joints.
Epoxy grout works great to resist stains but it may be a bit difficult to apply.
You will be butting the tiles up against each other so there isn t any.
Perfectly cut edges will not perform better than regular tile when friction occurs so they can still cause a variety of problems if you opt for a no grout tile installation.
In most other tile i will use 1 16 or 1 8 inch lines.
Both marble and grout are easy surfaces to clean when you apply the proper cleaning routine.
Many homeowners prefer the appearance of ungrouted tile.
This insures that there is no bleed through effect of the ridges in the thinset you spread on the surface.
Even if rectified tiles allow you to avoid using grout without having the result look unaligned the materials are still in danger of being chipped or damaged.
Therefore in order to get the grout lines to match you may need to cut either the floor or the base board tiles which can add considerable cost to your project in terms of labor and material.
Laying a marble floor without grout is possible so long as proper care is taken to assure your layout is perfectly square.
Before installing your marble tiles without grout consider the function of grout and whether it is appropriate to eliminate the step.
It is important to find ingredients that will clean effectively yet be mild enough to cause no damage to either surface.
For most granite and marble tile i install i use either 1 32 or 1 16 inch grout lines.
I try to use the smallest grout lines the tile will allow.