A Great Lecture Outline -- covers the basic principles of light as well as in depth discussion of birefringence and even trirefringence, which I find really interesting. Hmmm. Maybe a project in their somewhere.
Site about Labradorite -- which above site lists as possibly trirefringent. From this site, information from a huge database of minerals can be accessed. Names like Oviline (Peridot) have caught my eye.