According to Nintendo, Diancie can create diamonds out of thin air and uses her diamond to attack and defend herself. Being a rock type is quite impressive since rock type attacks in pokemon are strong and only few types has resistance and double resistance to the type of attack. Paired with a fairy type, that would be an awesome combination to defeat your foes.
In regards to the date of its release, Nintendo haven't provided yet any information on how pokemon trainers could obtain Diancie but game developers had shared us a trailer of the pokemon. They also provided us a hint that the pokemon will start in the next movie as Ash try to search for the legendary pokemon Xerneas. Yveltal will also debut on this movie which is expected to be release this summer.