I confess, I had to ask Google to help me on this one. One of my best friends is really into gems and crystals, so it seems that I have a LOT more to learn from her!
Apparently, this is a hard question to answer since not everyone agrees on what “rare” should mean. However, there is a gem called musgravite (so rare, I hadn’t even heard of it!) that only has had about 8 specimens ever found (as of 2005)! The Gemological Institute of America (GIA) calls musgravite “a rarity among the rare”!