Each and every Pokémon has their name and appearance based off of something or another and Diancie is no exception. I'll admit doing the research for this Pokémon was fun and a bit interesting. Here are some cool things I found out about Diancie.
It is said that Diancie's appearance is based off a carbuncle and, well, to be honest when I think of a carbuncle I think of this little guy as seen on the left, but I think that's probably the wrong kind of carbuncle. It's still cute though! Instead wiki's say the carbuncle Diancie is properly based off of is more akin to a fairy creature with a gem on its forehead. I also checked on Google to find out more and it is a type of bright red gem, a garnet in particular, which does sound accurate due to all the diamonds found on Diancie. Carbuncle is also something else but I won't go there because it's gross and eww. Don't look it up. Diancie's appearance could also be based off of a princess-cut diamond due to its princess-like appearance.
As for Diancie's name it is likely a combination of several different words, the most obvious being diamond. Other possible words include fairy, fancy, pixie, and 石 ishi. I couldn't tell you which is the correct origins, but I'm partial to diamond and fancy just because I hear those words more clearly when I say its name aloud. Apparently though I say Diancie's name wrong. I pronounce its name as dee-ahn-see but I guess the correct pronunciation is die-ahn-sey. Meh, details. Even knowing this I am going to continue saying it wrong just because I like the sound of my miss-pronunciation better and I'd probably still say it out of habit.