Well, ok...But Rarity's cutie mark was to make clothing. Her horn's magic dragged her to a rock with gems in it, but you see that she didn't earn her cutie mark right away when the rock split in half to reveal the gems inside. After she added the gems to the dresses she made and was told she did an excellent job, that's when she earned her cutie mark. And a cutie mark is your talent, too, as Rarity's talent is to make clothing. You see her making dresses and such more than finding gems.Since Twilight Sparkle was happy to become an alicorn princess, in this picture I just wanted to say that she's thanking Rainbow Dash for creating a Sonic Rainboom so she and the other Mane 6 ponies could earn their cutie marks. Twilight's cutie mark is being good at magic, and apparently her cutie mark represents the meaning of life and everything and there is one main star which represents her, and 5 other small stars which represent the other Mane 6. Then, as you said, she became an Element of Harmony with the others, learned how important friendship really is, then this lead on to this Season 3 Finale when Twilight finished a spell that no other pony could've done (although I think Celestia or Luna could've solved it anyways) which showed how powerful she was, blah blah blah, and she became an alicorn princess.And Twilight was destined to become a princess because the writers planned she would become a princess in the series from the beginning.