Its just a bit of fun I suppose. Mine seems to fluctuate. 10 minutes ago it was a red gem. Now a green gem. Sometimes its grey. People give me them without putting a reason. Others say they just felt like it. Shouldnt be taken too seriously.
Ive found that a lot of people with more than 2 green gems on this forum are quite bland. I imagine their buddies (who are mentioned in their signatures of course) just give them positive rep for the sake of it with the favour being returned. Thats how we get some folks with gems coming off the page.
Just my take on things....