Alms Tax: Paying Zakat with Coins of Higher Alloy or Other Metals
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1922
If a person owns gold or silver coins that are mixed with a usual amount of alloy, there is no problem if he gives zakat on them in gold and silver coins that contain more than the usual amount of alloy in them, or in coins that are not of gold or silver. However, in such a case, the value of the coins he pays in must be equal to the value of the zakat that is obligatory for him.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)