Mineralogy
The Gemology Project is a Wikipedia-style database devoted to the identification of gemstones. It contains articles on the fundamentals of gem and mineral identification as well as details of numerous techniques and pieces of equipment used in identification, and articles on the properties and uses of numerous gems and minerals.
This mineral database contains 4,442 individual mineral species descriptions with links and a comprehensive image library.
A website describing the facilities at the University of Minnesota microprobe lab. The images page is well worth a visit.