Well if you didn't use a tile sealer on the stone before you installed it, then the stone tile would be porous and the grout would have sucked into the stone. You can purchase a jug of muriatic acid and apply it to your tile, "thoroughly soaking it", let it sit for 2 or 3 minutes then take a scrub brush and give it a good going over. Applying more muriatic acid if necessary. Read the directions on the jug. I've never tried it on stone before but it works on ceramic and porcelain floor tile. If you used a sealer then you can also try using hot water and a razor scraper. Sorry I couldn't be of more help. Good luck to you.