Patch 2.1.0 (22-May-2007): Essences (Earth, Fire, Air, Water, Living) have had their drop rates significantly increased. Also Outland creatures that dropped motes will now have a chance of dropping essences as well.
Patch 1.3.0 (07-Mar-2005): Changed to good quality (green) to indicate their value and so that they are distributed properly by group loot.