Sapphire

Sapphires and Rubies are probably the rarest crafting materials in the game. They are even harder to find then Ectoplasm. They are usually found in the Underworld or Fissure of Woe, but there have been reports of them found in other areas of the game too, such as Sorrow's Furnace.

It is believed by most players that some future expansion will contain new armor sets, which will require these crafting materials. For this reason, people have already begun hoarding them, and the prices on the NPC traders for these materials can be very high.