A very rare ore from which Omega molecules can be synthesised, a process attempted by the Borg and later achieved on a larger scale by an alien research team in the Delta Quadrant.
Boronite is the raw material for making the Omega molecule, the most powerful and dangerous substance known to Starfleet. The Borg came to believe that Omega could be created from boronite ore, and their attempt held a single molecule stable for a fraction of a second before it destroyed dozens of ships.
In the Voyager episode The Omega Directive, the ship detects an Omega explosion at a research outpost where alien scientists had processed a rich boronite deposit into hundreds of molecules as an energy source for their struggling civilisation.
Janeway, bound by orders to destroy Omega wherever it is found, has the molecules beamed into a harmonic chamber designed by Seven of Nine and eliminated, despite Seven's wish to study them.