Diancie is a gray, sylph-like Pokémon with a white chest and arms that resembles a dress. It has a headdress which consists of pink crystals, four short ones cresting along top pointing upward and two long ones on either side of its head pointing downward, with a diamond-cut centerpiece framed by its slim gray ears. A smaller pink crystal is also embedded in the back of its head, and a gold nugget is embedded in its forehead. Its eyes are red with pink irises. A gold crescent circles its neck, with the ends joined by a rhomboid gem. Its lower body resembles a large oblong chunk of stone, with a large pink crystal protruding from it.
Mega Diancie retains Diancie's color scheme but instead possesses a dress-like structure made of pink gems and a larger chunk of gem replacing the large chunk of rock. The rhomboid gem on the back of its head becomes rounded and two smaller gems grow from it at the bottom. More gold pieces support the large diamond at the lower body. Its centerpiece becomes heart-cut instead of diamond-cut. It also gains two ribbon-like structures growing from the two small gems on the back of its head and are tipped by two small floating rhomboid gems on each ribbon. The two long pink crystals behind its head shrink into one and two more of these hang from the centerpiece and are also tipped by a small floating rhomboid gem each. A larger gold crescent also circles its torso. Because of its noble beauty, Mega Diancie it is commonly referred to as "the Royal Pink Princess".
Diancie is born from a Carbink that undergoes a mutation. It can instantly create diamonds by compressing the carbon in the air; these diamonds can then be used to defend itself. It can absorb light with a perfect balance and emits a radiance that looks like nothing in the world. According to legends, it can protect its travel partners from accidents and illnesses. Diancie is the only known Pokémon that can learn Diamond Storm.
Diancie debuts in the X & Y chapter, as a silhouette mentioned by Gurkinn after the Tower of Mastery had been destroyed, with him musing over the fact the secrets of a certain Pokémon will be lost with it. It was first seen being desired by the Shabboneau Castle owner, who sent his butlers out to obtain Diancie for him. Diantha, Gurkinn and Korrina later encountered Diancie being captured by one of the butlers. The butler reveals he only worked for the castle owner to get information about Diancie and proceeds to attack the trio. The butler is eventually defeated by Diancie's Moonblast and is sent running away. After saying their goodbyes, Diancie leaves with its Carbink companions to find a new home.
Diancie also appeared in the Omega Ruby & Alpha Sapphire chapter, having helped Ruby and Latios from drowning after they were knocked into the sea. It is currently unknown if this Diancie is the same as the one in the X & Y chapter.
Tied with Deoxys, Diancie has the lowest HP base stat of all Legendary Pokémon.
Diancie could be based on the carbuncle, a mythical fairy-like creature that was rumored to have a jewel embedded on its forehead. Diancie could also be loosely based on a "princess-cut" diamond, referring to its princess-like appearance.
Diancie may be a combination of diamond, fancy, and 石 ishi (stone). It may also include pixie.