http://www.shef.ac.uk/chemistry/web-elements/
WebElements is an online periodic table at the University of Sheffield in England. Clicking on any entry on the table will bring up a very large amount of useful and interesting information about that element.

http://www.rockhounds.com/
Bob's Rock Shop, despite its name, is a noncommercial site featuring information for mineral collectors (rockhounds) as well as general mineral information and images of interesting and spectacular mineral specimens.

http://mineral.galleries.com/
The Mineral Gallery offers information, including physical properties, and images of minerals organized either alphabetically, by mineral class, or through a keyword search. While it is a commercial site, it is low-key and offers a large amount of useful information.

http://www.bris.ac.uk/Depts/Geol/opmin/mins.html
The Minerals Under the Microscope web page at the University of Bristol in the U.K. offers extensive information about the theory and practice of how geologists utilize polarized-light microscopes to identify and study minerals.

http://www.tannerm.com/atoms_and_elements.htm
Tanner's General Chemistry page offers a review of the basics of atoms and molecules.

http://www.uni-wuerzburg.de/mineralogie/links.html
Links for mineralogists from the Institute of Mineralogy at the University of Würzburg in Germany.