As of this time, Jewels are only obtainable through purchasing them from the Jewels Shop. You can purchase the Jewels by clicking on the + button located at the top of the game screen.
Jewels purchased inside the level can be used right away in the level that they were purchased in. Just click on the item that you wish to purchase after the Jewels transaction is complete to buy the item.
everyone starts with 10 jewels