Banalinga

Banalinga, a stone found in nature, in the bed of the Narmada river in Madhya Pradesh state, India, is an iconic symbol of worship, based on either the scriptures or cultural traditions among the Hindus, particularly of the Shaivaites and Smartha Brahmins. Stones are ancient and connote divinity. It is a smooth ellipsoid stone.
Banalinga is also called the Svayambhu Linga: (Sanskrit) "Self-existent mark or sign of God", as it is discovered in nature and not carved or crafted by human hands.
The forms of Linga can vary in detail from a simple roller shape roughly cylindrical Banalinga to the stone carved with a thousand facets (Sahasralinga) or of light relief in several human figures (Mukhalinga). The Linga in the shrine of a temple is in stone.