I experience the fingernail mark from time to time, even using the best cards (Fournier, Copag etc etc), on one occasion the deck was marked on 5 separate occasions at the same table before I finally halted play on the table and broke it, sending each player to a different table and recording where they went, I warned the players that the tables they were headed to had not had any problems with finger nail marks all day and that if a card 'suddenly' was found marked the same way after they moved that player would be disqualified from the tournament, low and behold now further marks were discovered or made to the decks in use. I usually have a spare of each colour in stock for replacing marked cards, it's very hard to prove who it is unless you can put a trusted ringer onto the table to analyse the way players are viewing their hole cards. I eventually found out who it was that was marking the cards and he has not returned to any events after the table breaking incident.
Regards
Stuart