Shila, (śila in IAST) or Shaligram refers to a fossilized stone used to invoke God, and as a representation of God. Shaligram is usually collected from sacred river beds or on the banks. Shiva worshipers use nearly round or oval shaped Shaligrams to worship as Shiva Linga.
Vaishnavas (Hindu) use aniconic representation of Vishnu, in the form of a spherical, usually black-coloured Ammonoid fossil found in the sacred river Gandaki.
They are more often referred to as Shilas, with Shila being the shortened version. The word Shila translates simply to 'stone' and Shaligram is a less well-known name of Vishnu. The origin of the name is traced to a remote village in Nepal where Vishnu is known by the name of Shaligramam. Shaligram in Hinduism is also known as Salagrama. The name Salagrama refers to the name of the village on the bank of Gandaki where the holy stones are picked up. The name is derived from the hut (sala) of the sage Salankayana, who beheld the form of Vishnu in a tree outside his hut (cf. Varaha-purana).