Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.049.

The rikaz are all those substances upon which one-fifth (khums) is payable, such as gold, silver, iron, lead, brass, and the like. This is the opinion of the Hanafiyyah, the Hanbaliyyah, Ishaq, and Ibn al-Munzhir. A report from Malik and one of the two opinions of ash-Shaf’i also corroborate it. Ash-Shaf’i also holds that only gold and silver are subject to khums.