Rubies seem the hardest items to find. Here is my tip for beginners:
-First boss is the fastest way to harvest, it takes me less than 2 minutes per kill/2 items.
-Wipe quickly the 3 levels with a magician (anything except poison that might kill the boss).
-At the third level finish the boss with an explorer with Pickpocket. You receive 2 items.
-The third spot depends on your level, either another magician if you have difficulties wiping the mobs quickly or a second explorer for the boss.
-Expect one hour of grinding (30 kills) for 4 rubies and more, it’s more hand picking than mechanized harvesting… but well :)