You let go of them gem over your Socket, on your face :3

After that, you should be able to use LMB to cast fireballz.

You can also stash the gem in your bag, but that sadly has no purpose at the moment, as we didn't have time to implement more gems into the game. If we did, you would've used the bag to carry all of them with you, and switch between them depending on the situation .w.

We didn't have time to implement a tutorial or anything, sorry :<