The basics.

Name: Diancie (English), ディアンシー Diancie (Japanese)
Type: Rock/Fairy
Species: Jewel Pokémon
Pokédex: #719 (National), #151 (Central Kalos)
Height: 2'04" / 0.7 m (3'07" / 1.1 m when Mega Evolved)
Weight: 19.4 lbs. / 8.8 kg (61.3 lbs. / 27.8 kg when Mega Evolved)
Abilities: Clear Body, Magic Bounce (Mega)
Damage taken: Is weak to Ground, Steel, Water, and Grass-type attacks; takes normal damage from Fighting, Poison, Rock, Ghost, Electric, Psychic, Ice, and Fairy-type attacks; takes half damage from Normal, Flying, Bug, Fire, and Dark-type attacks; is immune to damage from Dragon-type attacks.

Diancie is a generation VI Pokémon which makes its debut in Pokemon X/Y for the Nintendo 3DS. It is a Rock/Fairy-type Pokémon and under normal conditions does not evolve into or from another Pokémon. However, if holding a Diancite it can Mega Evolve into Mega Diancie during battle. Diancie is considered a Mythical Pokémon due to its rarity and difficulty to obtain. It is also an event exclusive Pokémon and can only be obtained during promotional events and is unobtainable through normal gameplay. Diancie is the only Pokémon that can learn Diamond Storm and is the only Mythical Pokémon that can use Mega Evolution.

Physical appearance.

Diancie's physical appearance is something of a fairy. Although genderless it looks quite feminine. This could explain why I often find myself referring to Diancie as a "she" rather than an "it" (even though I continue to call Diancie an "it" throughout the shrine). It has red eyes with pink irises, and its body is a sort of purple/gray color and has a rocky appearance, with its bottom more visible as a rock with a pink diamond sticking out of its left side. It wears a white shirt-like garment with white sleeves. On its chest is a diamond shaped pink gem with a sort of necklace around its neck which is colored yellow. On its head is a giant pink diamond with a small gold gem underneath and four larger pointed gems sticking out the top, with two longer gems sticking out downward sorta to the sides.

Diancie's Mega Form is simply elegant and quite the sight to behold. The colors on Diancie's Mega Form remain unchanged from its normal form, however, its shirt-like garment has lost its sleeves, exposing its arms, and has gained an additional ring on its waist. Its bottom part is now a pink diamond and has several long pink diamonds flowing behind it with two smaller diamonds hanging off the end of each one. It also appears to have a transparent skirt on the inside of the diamond skirt. The diamond on its forehead has a slight heart-shape and appears larger. The pointing upward diamonds appear smaller and there are now three diamonds pointing downward (two in the front and one on the back) which are also smaller, but elegant like the bottom diamonds and have a single diamond hanging off each one. From the back of its head flow two long transparent ribbons which reach down to Diancie's base. On each corner also hangs two diamonds.

Although quite rare, Diancie does in fact have shiny colors. However, I have no idea how you'd get them seeing as you can't breed Legendary or Mythical Pokémon. Maybe through events or soft resetting? I don't know. Anyway, Diancie's shiny colors are somewhat different. While the color of its pink diamonds remains unchanged its shirt becomes a vivid pink. Its body also becomes a darkish gray. Other than that nothing else has really changed from what I can see, even in its Mega Evolved form. Sorry to disappoint!