Recover all three gems and a power source for the necklace from Uldaman, and then bring them to Jarkal Mossmeld in Kargath. Jarkal believes a power source might be found on the strongest construct present in Uldaman. From the journal, you know... * The ruby has been stashed in a barricaded Shadowforge area. * The topaz has been hidden in an urn in one of the Trogg areas, near some Alliance dwarves. * The sapphire has been claimed by Grimlok, the trogg leader.
Here is the journal, translated for you to read on your own. The crux of it says dwarves hid the ruby, the troggs hid the topaz, and a trogg named Grimlok has the sapphire.
Look - you're going to need a power source to make the necklace work again. Dran Droffers doesn't know this. To find one, here's what I'd do: go as deep as you can in Uldaman and kill whatever construct there is the strongest. No doubt whatever that is would have it.
When you have everything collected, come back here!