[Is Khums Obligatory on Stones That Are Sold?]

Answer—and from Allah is success: What has come from the Prophet—May Allah bless him and his family and grant them peace—in this chapter is his saying: “... and in buried treasure (al-rikāz) there is one-fifth (khums).” The scholars have explained this as meaning what Allah the Exalted deposited in the earth of the minerals of gold and silver.
And it is related from ʿAlī—peace be upon him—that a man found a quantity of gold in a ruin, and he commanded him to pay one-fifth of it.
And He, Exalted is He, said: “And know that whatever you obtain as spoils of anything—then indeed, for Allah is one fifth of it and for the Messenger ...” [al-Anfāl:41]. On the basis of these foundations we say:
First: It is necessary to consider and identify the effective cause that brings about the obligation of khums in rikāz.
It is possible that the cause is: that the wealth has been embedded (marqūz) in the earth since the day Allah created it. This is an instance of assigning a ruling to a descriptive attribute, like: “A deliberate killer does not inherit.”
On this basis, khums is due on everything a person extracts from the treasuries of the earth that counts as significant wealth—such as the various kinds of iron, copper, aluminum, glass, petroleum, gemstones, and the like.
As for water, soil, gravel, and building stones—if their prices rise—khums is not due on them. Likewise, the stones from which tiles are made carry no khums—except that there are stones for which khums is due, such as the variously colored agates (ʿaqīq), and some clear quartz stones (marw) from which beads are made to adorn garments and necks.
We have excluded water, soil, and stones because they are the very substance of the earth; khums is established only in what Allah the Exalted has embedded in the earth of wealth since He created it.
Moreover, the dearness in price of water, soil, gravel, and stones is incidental. They may have no value in their place, and only acquire value when transported to a land far from their source. It may be that the price for which any one of them is sold covers nothing beyond the cost of transport, laborers’ wages, and their provisions.
As for firewood, herbage, and game—of land or sea—these do not fall under the term rikāz.
— As for the gems extracted from the sea, khums is due on them, for they have been created in the earth since the earth was created.
— As for His saying, Exalted is He: “And know that whatever you obtain as spoils of anything—then indeed, for Allah ...” [al-Anfāl:41], it may be interpreted as follows: the term ghanīmah (spoils) potentially carries two meanings:
1 - That it is, in the Sacred Law, a term for what is taken as spoils from the wealth of enemy combatants. If so, then khums would be specific to that.
2 - That it remains upon its linguistic meaning, thus encompassing what is taken as spoils from enemy combatants, and also what is taken from game—land and sea—firewood, herbage, and the like.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1