So, when the sun rose on the next day, Flurry Heart approached the Crystal Heart and- just as the book said- it glowed as she saw a reflection of herself, but… Older, and in a glass pane similar to the ones she's seen in Canterlot Castle. Feeling reassured, Flurry Heart bowed to the Crystal Heart- promising to protect Equestria alongside the other princesses. The Crystal Heart shone brighter than ever before, a snowflake-shaped pattern appearing on the ground and even Flurry began to glow. In a burst of light, the Crystal Heart shot an aurora into the sky and in a similar manner, towards Flurry Heart, and…

There was her Cutie mark.