Several years ago, I had about 700 sq ft of tumbled travertine tile put in
(the earth tone/beach sand color) with bone (beach sand color) grout.
Between a few sloppy contractors that came in afterwards + over washing, the
grout turned almost black in areas. It was sealed when it was installed, but
never afterwards. I called in a guy to clean it, but he said it would come
out nicer and "new looking" to skim coat it with a very thin coat of grout
to sort of dye the grout back to the original color. BIG MISTAKE that I went
this way. Well, sort of... after it was done, yeah, it look brand new for
the most part. But you guessed it... the thin coat of grout soon started
crumbling off.

So now I'm stuck with over filled grout lines that are half crumbling off.

Any way to save the tile? or does it need to be replaced?

As I said, its tumbled, so the surface of the tile is filled with grout too,
you can't exactly just grind out the grout lines.

Is there perhaps some kind of chemical or acid they use to strip just grout
out or something? to take the tile down to its original ungrouted form?

Thanks.