Shaligram

fossilized shell, typically in the form of spherical, black-coloured Ammonoid fossils of the Devonian-Cretaceous period, used in Vaishnava Hinduism as aniconic representations of Vishnu, usually collected from river-beds or banks

Shaligram asscosiated with lord vishnu ,the presever in Hindu mytholy is workshiped as vishnu itself moslty during tulsi pooja , shaligram and tulsi are married to each other as it tends to bring prosperity, happiness and blessings in family .