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.

Diancie is born from a Carbink that undergoes a mysterious mutation. It can instantly create diamonds by compressing the carbon in the air. Diancie uses these diamonds to both attack and defend itself. Diancie's pink, glimmering body is said to be one of the most loveliest sights to experience in the world. Diancie is also said to be one of the most beautiful Pokémon seen in the world. Diancie is found living in underground caves filled with diamonds such as the Diamond Domain, a crystal kingdom where it reigns as queen over many Carbink, as it has occasionally been seen traveling or living with others. As seen in the anime, Diancie can speak through telepathy.

Forms
Diancie can Mega Evolve into Mega Diancie.

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. The head diamond measures over 2,000 carats.

Thanks to the Mega Evolution energy, Diancie's manipulation over diamonds has grown so much, that Mega Diancie can now create a sword out of them during battle. Due to the impurities on the surface of body having fallen away, Mega Diancie can now shine so brilliantly that when reflecting light, one cannot look directly at it. In this form, it was known as the "Royal Pink Princess" due to its colors and appearance. Due to it being blessed with both beauty and skill, Mega Diancie can easily adapt quickly to any situation.

(Source: Bulbapedia)