Eridanium can usually be obtained at several planets scattered through space.
It is very strong, being one of the strongest materials in the game. The malleability is also good enough for it to be used for practical building. The only downside to it is the heavy weight.
Trivia
The only materials stronger than Eridanium are the Forcefield, EnergyShield and Arcolantium.
It was formerly called Naquadah.
During times when it was called Naquadah "UW2 UU1" a player named ryano supposedly invented a weapon of mass destruction called "naquadah bomb" using complex maze pattern made out of Naquadah and laid it out in it with explosives placed inside semi-randomly or in the center.