Nautilus. In this case, a fossilized Nautilus whose diameter is more than 3 feet! They grew them BIG back in those days!
Negative. Ha! Your brain is probably saying, "Hey, that's not a negative, but the face of Buddha!". But, Grasshopper, it is a negative of the face of Buddha.
Noni. Popular today for its phytochemicals. Grows just about everywhere in Hawaii (and elsewhere, but I haven't been to that many else wheres). A good overview of Noni can be found here.
Nopal. Another plant with medicinal uses!
There you go for my submissions for the letter, "N". Do stop by ABC Wednesday to see what others have come up with!