Ore grade is the measured concentration of a valuable mineral or metal within a given mass of ore, expressed using units appropriate to the commodity in question. Iron ore grade is reported as a percentage of iron content (% Fe), with high-grade direct shipping ore typically exceeding 58 to 62 percent Fe. Bauxite grade is expressed as a percentage of available alumina (% Al2O3), with consideration also given to reactive silica content, which negatively affects refining efficiency. Gold grade is reported in grams per tonne (g/t) or, in some jurisdictions, ounces per ton, with underground gold mines often requiring grades above 3 to 5 g/t to be economic, while large open-pit operations can profitably process ore below 1 g/t due to scale. Diamond grade is unusual among these commodities because it reflects both the quantity and quality of stones recovered; it is most commonly expressed as carats per hundred tonnes (cpht) or carats per tonne, but overall deposit value also depends heavily on average stone size, clarity, and color, meaning two deposits with identical carat grades can have vastly different economic value.
