The World Federation One Stop Fiqh

Ruling 1921

If gold or silver coins contain a more than usual quantity of alloy, then, if they can be called gold and silver coins and they reach the niṣāb, zakat on them is obligatory even if the pure quantity does not reach the niṣāb. However, if they can no longer be called gold and silver coins, then zakat is not obligatory on them even if the quantity that is pure reaches the niṣāb.