This is a stone and is found only the in the river Gandald Mukiinahh of Nepal. It occurs the Sudarshan Chakra of Lord Vishnu and there is no idol of Lord Vishnu more sacred considcrd in this world. In these some are open and the chakra is visible and some are closed and have mini-holes on surface.Why there is a Chakra in this stone, it has a mythological theory. Sati Vrinda cursed to Lord Vishnu to be a black-stone and insects ate him. So insect lived in it and ate it and made cut-marks and left particles of gold. This is a wonder of nature and so the worship of Saligramji is considered to remove all the difficulties and pains of life. The worship of it brings name and fame and prosperity. The batting-water of it destroys the sinful thoughs and a daily worshiper goes to Bakunth Dham (the home-folance of Lord Vishnu). Keep it in y our home and worship it as a deity-idol.