Magic Smile indeed.

How come I see what looks like a Soul Gem on the right? It doesn't seem to be mentioned in the Pixiv tags either.

It's her wing crystals. They're just fancied up a little more than usual.

Not what I meant. Look at the wing crystals on the left side of the picture, then look at the blue one on the right. See a difference? What does the blue one look like to you?

From the thumbnail, I thought her eyes were like (◕ ◕)
Also

Paragon21XX said:
Look at the wing crystals on the left side of the picture, then look at the blue one on the right.

"Look at your hand, now back at her. She has it, it's your soul gem."