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The Ounce
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An ounce is an imperial unit of measurement equal to one sixteenth of an avoirdupois pound. The troy ounce (oz t), which is one twelfth of a troy pound, has prevailed as a unit of measurement for the pricing of noble metals,

and is equivalent to the apothecaries’ ounce equalling exactly 31.1034768 grams. It refers only to the pure gold content, which means that impurities have to be deducted from the measured weight.