Why isn't there a set of 5 rings? (It busts up our pretty set of odd numbers.)
Because the Ents didn't want any and nobody asked the Hobbits, that's why. They only made rings for 'important' races. ;-)
NewCenturions, in one of Tolkien's published Letters he said that Elves have slightly pointed ears. The word for "ear" and "leaf" are related, indicating that there's some connection between the shape of an Elf's ear and a (presumable pointy) leaf. Hope that helps!