Gemological Institute of America
(for information on diamonds and diamond grading)

International Colored Gemstone Association:
Gem Variety List
(detailed information on specific gemstones by variety)

Friends of Jade
(a group for jade lovers and collectors- includes articles and reference materials as well as online discussions)

Fred Ward Gem Book Series
(Opals, Jade, Emeralds, Rubies & Sapphires, Diamonds, Pearls, or Gem Care- an excellent read for each subject- also available at Amazon.com)