Following Diancie’s announcement in Japan via CoroCoro earlier this week, the english official site has revealed the mythical Pokémon as well. According to its Pokédex entry, Diancie is said to be created when Carbink undergoes a strange mutation, confirming a lot of fan theories. Diancie can create diamonds out of thin air by compressing the carbon in the atmosphere with its hands. It uses these jewels in battle to attack enemies and protect itself. Details about its distribution will be given at a later date.
You can check the screenshots as well as Diancie’s trailer below. Note Diancie’s shiny form in the first screenshot, it’s not clear yet whether it was revealed intentionally or not.