Is it permissible or forbidden to extract treasures claimed to exist beneath houses with the help of "the snatching jinn," as well as to sell what is extracted from them, such as "red and black mercury"?

It is not permissible to extract treasures through charlatans. What charlatans do are grave evils, some of which may even amount to Shirk (associating partners with Allah). The Prophet (peace and blessings be upon him) forbade going to a soothsayer or sorcerer and asking him questions. Whoever goes to him and asks him about something, his prayer will not be accepted for forty days. And whoever believes him has disbelieved in what was revealed to Muhammad. As for red mercury, it is an illusion among charlatans, and its existence has not been confirmed. If a treasure is found, and it was stolen, it must be returned to its owners or given as charity. If it was buried during the Islamic era, its ruling is that of a lost-and-found item (Luqatah) which must be publicized for a year. If it was buried during the pre-Islamic era (Jahiliyyah), then it is considered a 'Rikaz' (buried treasure).