Here you are, <name>. The gem only needs to be pressed against one of the pedestals near the base of the tower to activate its defenses. Use it when you're ready to enter the tower and find Trelane's remaining items.
Good luck to you. I'll be awaiting your return here at the camp. Oh, and speak to Skuerto before you leave. He'll give you the rest of the details you'll need.
If there are no other delays, how about we get started?