I think that people should be free to choose the gemstones that mean the most to them. Often, people assume that a diamond is what you "have to get", but if you allow yourself to choose among every gemstone available, then you can end up with a ring that is very "you" and original. This site is useful when choosing among gemstones: Gemstones. Information for Gemstone Jewelry Shopping.

There is also an interesting section about engagement rings.

Best of luck!