From your dungeon, open Defense Logs (dagger icon), find the player who has stolen your gem and tap "Retrieve" button. You will then get a chance to return your gem. Note, that you are not stealing a real gem, but the copy of this gem.
2. Call friends for help
If you fail to retrieve your gem, you will pop up in the Retrieve Logs (shaking hands icon in the bottom of your dungeon). Your friends will see that you need help and may try to steal your gem back.