Neutronium is an element vital to Asgard technology. It is also essential to the creation of Human-form Replicators. Orilla is particularly rich in neutronium, which is why it was chosen by the Asgard as the planet on which to rebuild their race after the destruction of Othala. (SG1: "New Order, Part 2")

Another planet rich in neutronium is Asuras in the Pegasus galaxy, a fact which the Atlantis expedition used to destroy the Asurans during the Battle of Asuras. (ATL: "Be All My Sins Remember'd")

Stargate Command has also started to mine some neutronium which was later stored on Earth most likely for the production of the Nanite medical program. (ATL: "Outcast")