I've been thinking... and that's not a good sign. 

But could it be that Rarity has some crystal pony connections?

I literally have no evidence other than she likes crystals and precious rocks, that's how she got her cutiemark, and her hair is "special", it has similar gradient to that of crystal ponies.

Which you could argue that many other ponies have that gradient but in fact there's not that many. Normally the pony's hair is plain.

 (No gradient)

The only others that I remember seeing with this kind of gradient aside from the crystal ponies are Sassy Saddles (which could be just hair dye), Cadence (the Crystal Empire princess), and Luster Dawn (arguably also Crystal pony descendant).

Though, none of her parents have any crystal pony attributes but it could still be passed on.

Of course, any connection is probably purely coincidental because I don't think any person could have had this always planned, that would be ridiculous, BUT it's fun to think about and it somewhat makes sense. 

So there you go.