A free online gemology school and reference site with well-categorized tutorials.
Your Gemologist - Gemology, Gemstones, Minerals And More
The National Museum of Natural History offers a massive online galley of minerals and gemstone photographs. There is also an useful search function.
Smithsonian National Museum Of Natural History - Geo Gallery
Would you like a feel of how and what students in big-name universities learn? The department of Earth & Planetary Sciences of UC Berkeley has freely made their lecture notes and course material available to the public.
University Of California Berkeley - Gems And Gem Materials