Neutronium
Neutronium is a rare, dense mineral, found naturally in the cores of neutron stars. It was impossible to scan inside neutronium. (; ) Despite many readings and theories, the Federation was never able to produce neutronium artificially as of 2375. ()

In 2267, the sensors of the  recorded a pure neutronium hull on the extra-galactic planet killer prior to its destruction. According to Spock, there is no known way of penetrating neutronium. ()

In 2366, Doctor Paul Stubbs studied the cyclical decay of trace amounts of neutronium in the Kavis Alpha sector through the use of the probe "The Egg." ()

In 2369, the  encountered the only known Dyson sphere. The structure was composed of a carbon-neutronium alloy and was impervious to phaser fire. ()

The Iconians had mastered the use of neutronium, and used it on a ziggurat containing a gateway on the planet Vandros IV. This was discovered by the Dominion in 2372 and was destroyed by a joint Starfleet-Jem'Hadar team before a group of rogue Jem'Hadar could use the gateway. ()

The Dominion had begun using neutronium by 2375 and used it to protect the entranceways on Dominion Headquarters from weapons fire. ()

By 3189, Starfleet had the ability to produce neutronium alloy. ()

Neutronium in the Delta Quadrant 
The crew of the  learned that the Borg never assimilated the knowledge to collect or create neutronium. ()

Separated from the Alpha Quadrant in 2371 Voyager encountered neutronium several times while stranded in the Delta Quadrant.

In 2371, Voyager encountered an artificial asteroid composed of a neutronium alloy created by the Vidiian Sodality. ()

According to an Alpha Hirogen hunter, at least one collapsed star in the Delta Quadrant had a neutronium mantle. The hunter claimed to the crew of Voyager that he tracked a silicon-based lifeform through this mantle. ()

In 2375, Voyager encounter the Think Tank, who traveled aboard a vessel composed of a neutronium alloy. This impressed the crew, who had rarely encountered it before. ()

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Geology
Elements