Taronite is not a missing square on the periodic table.
In real materials science, an element is defined by proton number. A grain is a discrete physical particle. A phase is a uniform region of material. An inclusion describes something distinct inside a host. A composite keeps distinguishable constituents together across interfaces.
Those words describe different scales and relationships. One microscopic object can sometimes be discussed as a grain, phase, and inclusion without becoming a new element.
Real meteorites can preserve microscopic presolar grains identified through isotopic evidence. That is a structural analogy, not proof.
In The Miracle Metal, Taronite is fictional: rare microscopic mineral or metallic grains incorporated into a larger composite. Its extraordinary properties belong to the story. The real science only gives the impossible a disciplined shape.