http://www.howistart.org/posts/ruby/1
This is a well-written, fun post on building a gem from scratch, from popular Rubyist Steve Klabnik. And I mean "from scratch"... he even guides you through an interesting approach to getting Ruby on your unix machine.
I heard about this on Peter Cooper's Ruby Weekly mailing list, which is required reading for Rubyists, though I find a lot more excitement happening on his JavaScript Weekly. 😉