As chibi joked below "nice.... cutie mark". Though it really is. I've been surprised how many times I've seen Rarity artwork that's gorgeous and with amazing attention to detail, yet has the 3 diamonds in her mark either inverted or the arrangement totally off. Got it spot-on here.
But on the work itself, the lighting and shading are amazing, and the expression you've given her just makes it. And there's something you've done with that line on her underside that makes it stand out to me even more than the "plot shot", heh. Good stuff.