The Premier Membership Organization in the US
Website features searchable faceting articles, discussion forums, news and events, a searchable library of faceting diagrams, information about competitions and a list of certified faceters.
International Gem Society
IGS publishes articles on gems, gemology and faceting on a weekly basis. Many of their articles were written by Jeff Graham, and while some of these may be out of date, many of his insights are still useful. IGS also has price guides for gems and there are tutorials available (with an additional cost) into many aspects of gemology.
The Gemology Project
A non-profit gemstone and gem science wiki-style database for anyone interested in gemstones and gemology (gemmology). Information packed resource with lots of faceting diagrams.