Are you talking about unglazed porcelain tile?
Because thats what happened to me once. Apparently you have to seal them before grouting.
Bianchi

when ever I use rocks or shells I seal them or they stain

Thank you ladies.

I've sealed tesserae before, but it was because I had painted them. So I was not sure if it worked for porous stuff also.

So do you use the same grout sealer stuff to do this or what? I picked up some beach stones this past weekend.

I use my tile sealer,aquamix.It tends to peel off during grouting, and only does a so-so job with vitreous tiles.

Hazie

I've used Polycrylic for sealing.