It can instantly create many diamonds by compressing the carbon in the air between its hands.
– Pokémon Y Pokédex
Diancie is the only gen 6 Pokemon to get a mega form. The rock-like formation of its lower body has transformed into an ornate dress, giving it an even more princess-like appearance.
It gets Magic Bounce as its ability when it mega evolves.
Stat-wise, it more than doubles its Speed when it mega evolves. It also swaps its offensive stats with its defensive ones; normal Diancie has higher defenses than its Attack/Sp Attack, but Mega Diancie has higher Attack/Sp Attack than defenses.