One can refer to this related question to know which are the 5 "Swayambhu Silas" or what is meant by a "Swayambhu Sila".
Hindus believe that Gods are present(naturally,without any human interventions) in certain stones and fossils found in some sacred rivers.
It is believed that Pujas done to such Holy objects produce merits which are equal to million times the Punyas(merit) obtained by worshiping "ordinary "man made God's idols.
Since,they are "Swayambhu"(self manifested)(it is believed) that the elaborate rituals of Prana Prathistha involving the Avahana Mantras and Avahanadi Mudras are not required .
Shalagramas are fossils found in the beds of Gandaki river ,Nepal and they represent Lord Vishnu.
Similarly ,Banalingas,found in Narmada riverbeds(in Madhya Pradesh) are considered as the corresponding "Swayambhu" forms of Lord Shiva.
But are these mere Hindu beliefs or are there confirmations available of these beliefs from Scriptures?