How do I remove a Permanant Marker line from a Polished Porcelain Tile?

I used a Permanant Marker to mark out the lines on Several Grey Polished Porcelain Tiles (for cutting via an Electical Tile Cutter).

Although I managed to scrub away the top layer of the Permanant Marker lines (following the installation of the Tiles) Part of the Marker seems to have been absorbed into the tile (below the polished surface).

Any ideas on how to remove it ?
Ive tried the following:
1. Various Tile cleaners
2. White Spirit
3. Nail Polish Remover
4. Vinegar
5. Paint Stripper
6. Various Household Cleaners (Including Bleach, Ciff, Fairy..)