The Gem of Impulse grants the user a "mana boost" of 100 points for somewhere around 3 hours, and is made from a crystal nodule that contains a baby Knath.

  Go to Xarabydun, the underground city, and enter the dungeon there.  Down at the bottom of the dungeon is a room with the nodules on the floor.  Get one and bring it up to the lady at the surface and she will give you the gem.  You can only make one gem per week, so don't grab too many.  Here are the complete set of maps for this dungeon: