Natural magnesite (magnesium carbonate in its mined, unprocessed state) is classified separately from calcined, fused, or dead-burned magnesia, which are covered under a different heading. The key distinction is whether the material has undergone thermal treatment: raw magnesite retains its carbonate chemistry, while calcined or sintered products have been converted to magnesium oxide. Invoice descriptions and chemical analysis certificates are essential to support the correct classification.

For refractory-grade use, buyers typically specify minimum MgCO₃ content (often above 90%) and maximum allowable levels of silica, iron oxide, and calcium oxide, as these impurities reduce refractoriness and can cause premature furnace lining failure. Agricultural and chemical-grade magnesite tolerates wider impurity ranges and is priced accordingly.