Whoever finds ore of a fusible precious substance on unclaimed land owes one fifth on it, and the remainder is his.
تحفة الملوكZayn al-Dīn al-Rāzīفصل في المعدن والركاز
Chapter on mined ore (maʿdin) and buried treasure (rikāz)
If he finds it in his house, nothing is due on it—unlike buried treasure.
If he finds it on his own land, there are two narrations.
ومَن وجدَ كَنْزاً ففيه الخمس ولو كان متاعاً والباقي لقطة في الضرب الإسلامي، وفي الجاهليّ هو للواجد إن كانت الأرض مباحة، وإن لم تكن فلمالكها أول الفتح، فإن جهل فلأقصى مالك يعرف في الإسلام، فإن خَفِيَ الضرب جعل جاهلياً.
Whoever finds buried treasure owes the fifth on it, even if it is merchandise; the remainder is found property under Islamic jurisdiction; in pre-Islamic custom it was for the finder if the land was unclaimed, and if not then for its owner at first conquest; if unknown, for the remotest owner known within Islam; if the jurisdiction was unclear, it was ruled as Jāhiliyya.
ولا شيء في الفَيْروزَج، والياقوت، واللؤلؤ، والعنبر.
Nothing is due on turquoise, ruby, pearl, or amber.
وفي الزِّئبق: الخمس.
On mercury, [due is] one fifth.