Extruded Underfoot Diamond Tradeskill Recipes for 300 - 350
Extruded Underfoot Diamonds are used for 7 different Jewelcrafting recipes and are a rather rare drop (I'd estimate .10 - .15%) off enemies that are level 50 - 90. Despite the item being added in during the Underfoot expansion, the diamonds actually have a chance of dropping off enemies as far back as Planes of Power!
Due to the extremely low drop rate of this item there is no good location for you to actually farm it, you best bet would be to go to a place in The Serpent's Spine, Secrets of Faydwer, Seeds of Destruction or House of Thule and hunt mobs in mass. In my personal experience these diamonds have a higher chance of dropping off higher level enemies, using my drop percentages above I would call PoP enemies a .01 drop rate where as SoD/Underfoot enemies would be a .10 - .15% drop rate.
Luckily though it's not all that uncommon to find these for sale in The Bazaar, even on low pop servers! Since these items have the same icon as a diamond and even have diamond in the name I've found most people will save them and try to sell them in The Bazaar.
Jewelcrafting Extruded Underfoot Diamond Recipes (7 Recipes)
Half-Moon Cut Underfoot Diamond
Marquise Cut Underfoot Diamond