ugh, her talking about it "gently rasping" her fingers reminded me of this book, Reef of Death by Paul Zindel. A few scenes where the baddies drop cages over victims heads with these killer snails in them that quickly rasped away flesh.
Popular alternatives to
Reef of Death
His Dark Materials 3-Book Paperback Boxed Set: The Golden Compass; The Subtle Knife; The Amber Spyglass