Boronite

From Memory Alpha, the free Star Trek reference.

Boronite is a rare mineral ore which is used in the synthesis of Omega molecules. The Borg were able to synthesize a single Omega molecule using a quantity of boronite, but they were unable to repeat the experiment as they never found another source of the ore. (VOY: "The Omega Directive")

A Vostigye space station destroyed in 2373 by astral eddies was constructed from sarium, boronite, and carbon-60 composites. (VOY: "Real Life")

In current knowledge, boronite is a copper-iron-sulfide ore that has a dark reddish-brown color with an iridescent surface film of tarnish. Perhaps the rarer future boronite contains scientific properties we cannot distinguish of common modern boronite. However, this seems unlikely; since boronite is quite a common substance on Earth, and since it is mentioned that boronite is rare, perhaps another substance is designated "boronite" in the future.