Diancie is a small Pokémon with a vaguely humanoid appearance. Its lower body is a conical, gray rock with a large, pink diamond in the side. Its upper body has two arms and is covered by a white material that resembles a dress. Its small, mitten-like hands poke out of the sleeves. Around its neck is a golden, half-circlet with a pink diamond in the center. Its head is gray with purple eyes that have red sclerae and pointed ears. On top of its head is a collection of pink crystals forming a headdress. There are four spike-like crystals along the top, a long crystal on either side hanging downward, and a large circular centerpiece on its forehead. There is a small, golden piece hanging off the center crystal and a small pink diamond embedded in the back of its head.
As Mega Diancie, its white dress becomes longer and loses its sleeves. Its lower body turns into a pink crystal and is surrounded by a series of long crystals similar to those on its headdress. On the back, both the dress and the crystals are longer, and additional, smaller crystals are suspended from the tips of the larger ones. The tips of the circlet around its neck curve outward and there is an additional half-circlet around its waist. The centerpiece of its headdress is now heart-shaped, and the remaining crystals are slightly smaller. The crystals hanging from the sides of its head have additional crystals suspended from the tips, and there is an additional hanging crystal on the back of its head. The diamond on the back of its head becomes rounded and two smaller gems form at the lower corners. Two ribbon-like structures grow from these additional gems; each ribbon has two small crystals suspended from the tips. Mega Diancie is known as the "Royal Pink Princess" due to its noble appearance.
Diancie is born from a Carbink that undergoes a mutation. It can instantly create diamonds by compressing the carbon in the air. Diancie is the only known Pokémon that can learn Diamond Storm.
Diancie debuted in this chapter, where it was mentioned by Gurkinn as he was escaping the destruction of the Tower of Mastery in Heracross Transforms. It was sought after by the owner of the Shabboneau Castle, who sent his butlers out to obtain Diancie in PS569. One of the butlers, actually a thief, found Diancie and attempted to steal it. With the help of Gurkinn, Diantha, and Korrina, the thief was defeated and sent running away. Afterward, Diancie left with its Carbink companions to find a new home.
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 a visual pun derived from the idea of a "princess-cut" diamond, referring to its princess-like appearance, although the diamond on the forehead resembles a brilliant-cut in the base form and heart-cut in the mega form. Its relationship with Carbink may be inspired by eusociality, a biological phenomenon which usually manifests in a colony of animals having a sole breeding female, or queen, tended to by her usually infertile offspring, or an example of a highly rare and unique mutations such as albinism or polydactyly. Alternatively, the transformation from Carbink to Diancie could reference metamorphic rock formation.
Diancie may be a combination of diamond, fancy, pixie, and 石 ishi (stone).