Neutronium was a dense alloy made up of non-toxic, trans-uranic elements on the periodic table that had far more neutrons than protons or electrons and it was difficult to produce. Groups and races who have managed to manufacture and use it include the Dominion, the Iconians and the Vidiians.
It was a super-dense element that was found at the heart of a collapsed star, which made it a rare but extremely tough substance. As armor, it was resistant to almost any form of energy and matter. This made it impervious to all known forms of contemporary weapons. (FASA RPG module: A Doomsday Like Any Other)
A ziggurat on Vandros IV in which the Iconians kept a gateway was made of neutronium. In 2372, this forced the crew of the USS Defiant and Weyoun's Jem'Hadar soldiers to mount a ground offensive against the rebel Jem'Hadar on Vandros. (DS9 episode: "To the Death")
A particular Vulcan ship visiting the trading planet of Rigel IV was unable to sell its pure neutronium, and only made a profit after selling every single soft drink on board. (FASA RPG module: The Orions: Book of Common Knowledge)