Dropped off of - yes, none other than the first shrroms in DSP when you enter through from Neth Lair to the left. I was just pulling from there because of the ease of not having to worry about the adds (except for the wolves that pop right there). Then I found, low and behold something other than a stupid shroom... heh. But I would definately say it was a rare drop item.
I've looted these off many different Mobs in Tenebrous and Scarlet desert.I'm sure it's the same in all zones that have Luclin armor drops.If you have someone reliable to transfer these to one of your alts after you loot it,you might find it a handy thing,as these are lore and many of the classes require two of these for different pieces of thier Twilight Armor.
There are various jewels: Sun, Moon, Cloud, Meteor, Star, Astral, Sky
In Twilight Sea there are two NPCs for each class that has quests for armor and one weapon. Each quests require a specific jewel and two to three other pieces. The reward isn't always worth the headache getting the pieces are, but if it's better than what you are wearing now and you aren't a twink, you might trouble yourself to hunt around for the pieces you need.