Accept Wilfre's "Smashing!" quest from the 1st screen of /skytower. Walk up twice and accept Ithuriel's "Celestial Stones" quest. Defeat the 3 gemstones in the same room.
Also make sure to enable Quest Log Turn-Ins and Reaccept Quest After Turn-In from Advanced Options. This way you can keep your Quest Log open and turn in quests from there. You can access your Quest Log by pressing L.