Pretty straightforward: on the left is the dress that got ruined, on the right is the replacement.
At the time Rarity made the first dress, she didn't know Jade very well and was mostly concerned with making her look adorable. By the time she made the second dress, she was much more able to personalize it.
After she had finished the dress, gloves and ribbons, Rarity had Jade try the new creation on. The fashionista wasn't completely satisfied, because the outfit was incomplete in her opinion. She then had an idea. She dug around in her drawers and found a bag of jade beads. She proceeded to thread the beads onto a string and added a clasp to finish the job. When Rarity put the necklace around Jade's neck, she was happy with the little dragon's new look.
I didn't make the background, that honor goes to
Here's the original file: fav.me/d5mt4kb