1. Retrieve it yourself
From your dungeon, open the Defense Logs (dagger icon), find the player who stole your gem and tap the "Retrieve" button. You will then be given the opportunity to return your gem. Note, that you are not stealing a real gem, but a copy of it.

2. Call friends for help
If you fail to retrieve your gem, you will appear in the Retrieve Logs (shaking hands icon at the bottom of your dungeon). Your friends will see that you need help and may try to steal your gem back.