Mindat.org is probably the largest mineralogical reference on the Internet. It was started (as a DOS database) by Jolyon Ralph in 1993 - and was launched for free on the web in October 2000. Currently there are 9045 different minerals, varieties and synonyms listed, and information on 37149 mineral occurrences worldwide, from 6502 different sites. This is a great site to check out what mineral you have.
A to Z List
And ATHENA MINERALOGY List of Minerals Varieties
Gemstones - Contents
Keep Coming Back There Will Be More Links To Come Yet