GeologyRocks

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.

One of the worlds largest online mineral databases, with mineral and mineral location information, updated daily by members.

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.

If you have to look at thin sections, then you could do far worse than go to this page - a brilliant resource for those struggling (or just in need of a refresher) with thin sections.