Why does the zakat on gold and silver (one-quarter of one-tenth) differ from the zakat on rikaz (one-fifth), even though they are of the same class?

The fundamental principle for determining the obligatory amount to be extracted from rikaz (buried treasure) and the zakat on gold and silver is what has been authentically reported from the Prophet, peace and blessings be upon him. He mandated one-fifth for rikaz and took one-quarter of one-tenth from gold and silver. It is reported in al-Tirmidhi that the Prophet, peace and blessings be upon him, said: "The injury inflicted by an animal is not accountable; a mine is not accountable; and a well is not accountable; and for rikaz (buried treasure), one-fifth is due." And in Sunan Ibn Majah, it is reported that the Prophet, peace and blessings be upon him, would take half a dinar from every twenty dinars or more, and one dinar from forty dinars. The reason for the difference in the obligatory amount is the greater benefit from rikaz and the ease of its acquisition, as mentioned in Tuhfat al-Ahwadhi.