Yes.
A lot of people were speculating on this before, and it was confirmed a while ago when we got a glimpse at Diancie's Pokedex entry:
![enter image description here](/pokebase/qa-plugin/https-img-proxy/image.php?key=c85ed56c2b040431&url=http%3A%2F%2Fcdn.idigitaltimes.com%2Fdata%2Fimages%2Ffull%2F2014%2F02%2F14%2F14507.jpg)
As you can see, it states that it is a sudden transformation of Carbink, meaning it was a Carbink before. This doesn't mean Carbink evolves in to Diancie Though, Diancie is probably just a special occurence among Carbink.
Hope I helped!