Heres the most valid and truthful info, by wikipedia.
There is a purple chat gem that is only visible when inside a chat channel. There are many theories about the purpose of this gem. There is, however, no verified purpose for this gem aside from a meaningless message. The gem toggles between an active and inactive state. When activated, a chat message appears stating, "Gem Activated" and likewise, a message "Gem Deactivated" is shown when it becomes inactive. It is also known to say "Mooooooo!" and "Perfect Gem Activated" on random, rare occasions. The official Multiplayer FAQ noted that "[the gem] is working correctly and has more than exceeded our expectations". It wasn't until much later in a fan-chat where Bill Roper revealed that the secret about the gem is that it has no secret, "Sometimes a Gem is just a Gem!"
I doubt it does anything but I think it will be in diablo3 and it will do something. Blizzard likes to take things that the fans hallucinate and put them in the next game as an easter egg (such as the cow level in D1 and putting it in D2). I just hope it is something cool (like the cow level in D2 :)).