If you need more in-depth information, these sites have it.

Alessandro Da Mommio runs the outstanding alexstrekeisen.it. His site features comprehensive descriptions of rocks, minerals, and mineral phases, along with high-resolution photographs of minerals in thin section.

Siim Sepp’s Sandatlas is, in his words, focused on “the materials our home planet is made of.” His site features information on rocks, minerals, and (as you would expect) sand, illustrated with beautiful photographs.

Webmineral.com offers a wealth of information, including crystallography, occurrence, classifications, XRD spectra, and lots more.

Mindat.org offers all of what webmineral.com does, along with animated crystal diagrams and petrology information.