Diancie did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Diancie is a Rock/Fairy-typePokémon, originating from Kalos. It is the first of three Mythical Pokémon of this region, and appears to be formed from an unusual transformation of another Pokémon of Kalos, Carbink.
While it is not known to evolve to or from any other Pokémon, it can Mega Evolve into Mega Diancie using a Diancite.
Diancie is a gray, nymph-like Pokémon with a white, dress-like torso and arms. It also has a large, stone-like part underneath the torso, which is roughly the same height as the head and torso combined. Out of this rock protrudes a pink, diamond-like crystal. Its head has a headdress attached to it, which consists of the same kind of pink crystals and a golden embedment. The pink jewel on its forehead is round, and polished like a traditional polished diamond. On its back head are four more crystals, polished like the card suit Diamond (a rhombus), as well as two longer crystals that point more downward, with both of their ends placed under its ears. A crescent-like piece of gold circles its neck, and ends on the chest, where another crystal is attached, also shaped like a rhombus. Its eyes are placed right under the round jewel on its forehead, and have full red sclera and pink pupils.
A Diancie is apparently born from a Carbink's mutation, and thus is an abnormality under other Carbink. Even though many Carbink tend to gather around a Diancie as if it is their leader, Diancie pays little to no attention to them and only does what it wants itself.
A Diancie is able to create pretty diamonds in mere seconds by compressing carbon with its hands. It can use these diamonds to protect itself.
Diancie's relation with Carbink might possibly be a parallel to Manaphy's relation with Phione.
Diancie may be inspired by the carbuncle, which is a fairy-like creatures that possibly have a jewel attached to its head, said jewel also being called "carbuncle".
The name Diancie might be a combination of the words diamond, fancy, and 石 (ishi), stone.