Rarity was doing her best to hide her dresses and gems in order to keep them from being looted during the war. Then the mana surge hit, and Rarity became one with the gems. Her body became as beautiful, strong and rigid as a diamond, rendering her unable to move in anyway aside from levitating herself with her magic.
In fact, she'd found he magic had become much more powerful in this form, and she was able to use it to animate otherwise inanimate objects.
Art Credit: http://nyarmarr.deviantart.com/art/Rare-jewel-338097043











