Quarkium

A composite that exists in quark stars. It is actually not composed of free quarks, but rather a mass of diquarks and other quark particles, having likely collapsed from neutronium. It is (roughly) ten times as dense as neutron degenerate matter, and when it can be contained, it is generally used for similar purposes.

Hadronium capable of containing it is exceedingly rare, and the very potential for a free quark to stumble across it cause most such constructs to violently collapse.