Well. There is a slight problem with that request of where or how she got her chakra gem.
The Teen Titan Raven is based off of the DC Raven, but it isn't known which one. Not only am I implying universes, but also regenerations. Raven has been reborn a couple of times in the DC world, and it might happen here. Sometimes she had the gem, other times she didn't (I think, I'm more of a Marvel guy then a DC).
Anyways, the reason why is about the same, to help her keep her darker passions in check. It can be assumed, if we are following the "normal first Raven" which bears the most resemblance to the Teen Titain Raven, that it was affixed to her head sometime during her childhood. There was a event before where a monk knew who she was and attempted to seal her into limbo, but the spell backfired and killed him instead. It's hinted that it was Raven's power emerging. After that, another monk, Azar takes her under his wing and begins to train her. http://teentitans.wikia.com/wiki/Raven
You can thus assume that the Chakra gem was somehow, possibly through the uses of magic rather than Mimi's extreme method, attached to her head to control her darker side. In the comics, it was removed by Brother Blood who bit it off of her forehead.
Tenshi Nova wrote:I do, but why are you so against the fact she's wearing a chakra gem?
Mimi isn't wearing a legit chakra gem, but it is her chakra gem. It doesn't function the same or correctly.
It would be like if I had a model of a 69 Dodge Charger painted orange with a big 01 on the side and a Rebel Flag on the top. I still have a General Lee, but it isn't the General Lee.