During the GameSpy Developers Chat on May 11 2007, Eric Flannumwhispered "Buy mud bricks now!!!" before getting smacked by Ben Miller.
The question was "Can we expect rare materials such as: Onyx Gemstones, Diamonds, Bricks of Mud and Clay Bricks to finally make an appearance in the game?" and the official answer was "We can't answer that at this time because it will cause a financial crisis at the material trader. :)". Onyx Gemstones and Diamonds later found usage in crafting Destroyer Weapons.