Alms Tax: Gold and Silver Coins Containing Alloy
Practical Laws of Islam as per the teachings of Ayatullah Sistani
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.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)