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.
The only materials stronger than Eridanium are the Forcefield, EnergyShield and Arcolantium.
It was formerly called Naquadah.